diff --git a/README.md b/README.md
index 5d28cc0..9074298 100644
--- a/README.md
+++ b/README.md
@@ -647,6 +647,25 @@ custom_message.txt:1:8: code format error...
 
 NOTE: If there are more than one elements with error message instruction in a prioritized choice, this feature may not work as you expect.
 
+Change the Start Definition Rule
+--------------------------------
+
+We can change the start definition rule as below.
+
+```cpp
+peg::parser parser(
+  R"(
+    Start       <- A
+    A           <- B (',' B)*
+    B           <- '[one]' / '[two]'
+    %whitespace <- [ \t\n]*
+  )",
+  "A" // Start Rule is "A"
+)";
+
+parser.parse(" [one] , [two] "); // OK
+```
+
 peglint - PEG syntax lint utility
 ---------------------------------
 
diff --git a/docs/index.html b/docs/index.html
index 3ce55ee..edd068c 100644
--- a/docs/index.html
+++ b/docs/index.html
@@ -20,6 +20,7 @@
       <li><span>Source Code</span></li>
       <li class="editor-options">
         <ul class="editor-header-options">
+          <li class="option"><label>Start Rule: </label><input id="start-rule" type="text"></li>
           <li class="option"><input id="packrat" type="checkbox"><label>Packrat</label></li>
           <li class="option"><input id="auto-refresh" type="checkbox"><label>Auto Refresh</label></li>
           <li class="option"><button id="parse" class="parse">Parse</button></li>
diff --git a/docs/index.js b/docs/index.js
index 756258e..abe23d3 100644
--- a/docs/index.js
+++ b/docs/index.js
@@ -27,6 +27,7 @@ const codeAstOptimized = setupInfoArea("code-ast-optimized");
 const codeProfile = setupInfoArea("code-profile");
 
 $('#opt-mode').val(localStorage.getItem('optimizationMode') || 'all');
+$('#start-rule').val(localStorage.getItem('startRule') || '');
 $('#packrat').prop('checked', localStorage.getItem('packrat') === 'true');
 $('#auto-refresh').prop('checked', localStorage.getItem('autoRefresh') === 'true');
 $('#parse').prop('disabled', $('#auto-refresh').prop('checked'));
@@ -61,6 +62,7 @@ function updateLocalStorage() {
   localStorage.setItem('grammarText', grammar.getValue());
   localStorage.setItem('codeText', code.getValue());
   localStorage.setItem('optimizationMode', $('#opt-mode').val());
+  localStorage.setItem('startRule', $('#start-rule').val());
   localStorage.setItem('packrat', $('#packrat').prop('checked'));
   localStorage.setItem('autoRefresh', $('#auto-refresh').prop('checked'));
 }
@@ -75,6 +77,7 @@ function parse() {
   const codeText = code.getValue();
 
   const optimizationMode = $('#opt-mode').val();
+  const startRule = $('#start-rule').val();
   const packrat = $('#packrat').prop('checked');
 
   $grammarInfo.html('');
@@ -97,7 +100,7 @@ function parse() {
     'background-color': 'rgba(0, 0, 0, 0.1)'
   });
   window.setTimeout(() => {
-    const data = JSON.parse(Module.lint(grammarText, codeText, mode, packrat));
+    const data = JSON.parse(Module.lint(grammarText, codeText, mode, packrat, startRule));
       $('#overlay').css({
         'z-index': '-1',
         'display': 'none',
@@ -165,6 +168,7 @@ $('#code-info').on('click', 'li', makeOnClickInInfo(code));
 
 // Event handing in the AST optimization
 $('#opt-mode').on('change', setupTimer);
+$('#start-rule').on('keydown', setupTimer);
 $('#packrat').on('change', setupTimer);
 $('#auto-refresh').on('change', () => {
   updateLocalStorage();
diff --git a/docs/native.cpp b/docs/native.cpp
index 815682e..497940e 100644
--- a/docs/native.cpp
+++ b/docs/native.cpp
@@ -21,7 +21,8 @@ std::string escape_json(const std::string &s) {
 std::function<void(size_t, size_t, const std::string &, const std::string &)>
 makeJSONFormatter(peg::parser &peg, std::string &json, bool &init) {
   init = true;
-  return [&](size_t ln, size_t col, const std::string &msg, const std::string &rule) mutable {
+  return [&](size_t ln, size_t col, const std::string &msg,
+             const std::string &rule) mutable {
     if (!init) { json += ","; }
     json += "{";
     json += R"("ln":)" + std::to_string(ln) + ",";
@@ -43,11 +44,11 @@ makeJSONFormatter(peg::parser &peg, std::string &json, bool &init) {
 }
 
 bool parse_grammar(const std::string &text, peg::parser &peg,
-                   std::string &json) {
+                   const std::string &startRule, std::string &json) {
   bool init;
   peg.set_logger(makeJSONFormatter(peg, json, init));
   json += "[";
-  auto ret = peg.load_grammar(text.data(), text.size());
+  auto ret = peg.load_grammar(text.data(), text.size(), startRule);
   json += "]";
   return ret;
 }
@@ -64,7 +65,7 @@ bool parse_code(const std::string &text, peg::parser &peg, std::string &json,
 }
 
 std::string lint(const std::string &grammarText, const std::string &codeText,
-                 bool opt_mode, bool packrat) {
+                 bool opt_mode, bool packrat, const std::string &startRule) {
   std::string grammarResult;
   std::string codeResult;
   std::string astResult;
@@ -72,7 +73,8 @@ std::string lint(const std::string &grammarText, const std::string &codeText,
   std::string profileResult;
 
   peg::parser peg;
-  auto is_grammar_valid = parse_grammar(grammarText, peg, grammarResult);
+  auto is_grammar_valid =
+      parse_grammar(grammarText, peg, startRule, grammarResult);
   auto is_source_valid = false;
 
   if (is_grammar_valid && peg) {
diff --git a/docs/native.wasm b/docs/native.wasm
index 9b90119..dcdb1ea 100755
Binary files a/docs/native.wasm and b/docs/native.wasm differ
diff --git a/peglib.h b/peglib.h
index 8be3bcd..8ca797a 100644
--- a/peglib.h
+++ b/peglib.h
@@ -3298,18 +3298,15 @@ using Rules = std::unordered_map<std::string, std::shared_ptr<Ope>>;
 
 class ParserGenerator {
 public:
-  static std::shared_ptr<Grammar> parse(const char *s, size_t n,
-                                        const Rules &rules, std::string &start,
-                                        bool &enablePackratParsing, Log log) {
-    return get_instance().perform_core(s, n, rules, start, enablePackratParsing,
-                                       log);
-  }
+  struct ParserContext {
+    std::shared_ptr<Grammar> grammar;
+    std::string start;
+    bool enablePackratParsing = false;
+  };
 
-  static std::shared_ptr<Grammar> parse(const char *s, size_t n,
-                                        std::string &start,
-                                        bool &enablePackratParsing, Log log) {
-    Rules dummy;
-    return parse(s, n, dummy, start, enablePackratParsing, log);
+  static ParserContext parse(const char *s, size_t n, const Rules &rules,
+                             Log log, std::string_view start) {
+    return get_instance().perform_core(s, n, rules, log, std::string(start));
   }
 
   // For debugging purpose
@@ -3989,9 +3986,8 @@ class ParserGenerator {
     return true;
   }
 
-  std::shared_ptr<Grammar> perform_core(const char *s, size_t n,
-                                        const Rules &rules, std::string &start,
-                                        bool &enablePackratParsing, Log log) {
+  ParserContext perform_core(const char *s, size_t n, const Rules &rules,
+                             Log log, std::string requested_start) {
     Data data;
     auto &grammar = *data.grammar;
 
@@ -4023,7 +4019,7 @@ class ParserGenerator {
           log(line.first, line.second, "syntax error", r.error_info.label);
         }
       }
-      return nullptr;
+      return {};
     }
 
     // User provided rules
@@ -4081,7 +4077,25 @@ class ParserGenerator {
     }
 
     // Set root definition
-    auto &start_rule = grammar[data.start];
+    auto start = data.start;
+
+    if (!requested_start.empty()) {
+      if (grammar.count(requested_start)) {
+        start = requested_start;
+      } else {
+        if (log) {
+          auto line = line_info(s, s);
+          log(line.first, line.second,
+              "The specified start rule '" + requested_start + "' is undefined.",
+              "");
+        }
+        ret = false;
+      }
+    }
+
+    if (!ret) { return {}; }
+
+    auto &start_rule = grammar[start];
 
     // Check if the start rule has ignore operator
     {
@@ -4096,7 +4110,7 @@ class ParserGenerator {
       }
     }
 
-    if (!ret) { return nullptr; }
+    if (!ret) { return {}; }
 
     // Check missing definitions
     auto referenced = std::unordered_set<std::string>{
@@ -4129,7 +4143,7 @@ class ParserGenerator {
       }
     }
 
-    if (!ret) { return nullptr; }
+    if (!ret) { return {}; }
 
     // Link references
     for (auto &x : grammar) {
@@ -4153,10 +4167,10 @@ class ParserGenerator {
       }
     }
 
-    if (!ret) { return nullptr; }
+    if (!ret) { return {}; }
 
     // Check infinite loop
-    if (detect_infiniteLoop(data, start_rule, log, s)) { return nullptr; }
+    if (detect_infiniteLoop(data, start_rule, log, s)) { return {}; }
 
     // Automatic whitespace skipping
     if (grammar.count(WHITESPACE_DEFINITION_NAME)) {
@@ -4169,7 +4183,7 @@ class ParserGenerator {
       auto &rule = grammar[WHITESPACE_DEFINITION_NAME];
       start_rule.whitespaceOpe = wsp(rule.get_core_operator());
 
-      if (detect_infiniteLoop(data, rule, log, s)) { return nullptr; }
+      if (detect_infiniteLoop(data, rule, log, s)) { return {}; }
     }
 
     // Word expression
@@ -4177,7 +4191,7 @@ class ParserGenerator {
       auto &rule = grammar[WORD_DEFINITION_NAME];
       start_rule.wordOpe = rule.get_core_operator();
 
-      if (detect_infiniteLoop(data, rule, log, s)) { return nullptr; }
+      if (detect_infiniteLoop(data, rule, log, s)) { return {}; }
     }
 
     // Apply instructions
@@ -4189,9 +4203,7 @@ class ParserGenerator {
           const auto &info =
               std::any_cast<PrecedenceClimbing::BinOpeInfo>(instruction.data);
 
-          if (!apply_precedence_instruction(rule, info, s, log)) {
-            return nullptr;
-          }
+          if (!apply_precedence_instruction(rule, info, s, log)) { return {}; }
         } else if (instruction.type == "error_message") {
           rule.error_message = std::any_cast<std::string>(instruction.data);
         } else if (instruction.type == "no_ast_opt") {
@@ -4200,11 +4212,7 @@ class ParserGenerator {
       }
     }
 
-    // Set root definition
-    start = data.start;
-    enablePackratParsing = data.enablePackratParsing;
-
-    return data.grammar;
+    return {data.grammar, start, data.enablePackratParsing};
   }
 
   bool detect_infiniteLoop(const Data &data, Definition &rule, const Log &log,
@@ -4530,43 +4538,52 @@ class parser {
 public:
   parser() = default;
 
-  parser(const char *s, size_t n, const Rules &rules) {
-    load_grammar(s, n, rules);
+  parser(const char *s, size_t n, const Rules &rules,
+         std::string_view start = {}) {
+    load_grammar(s, n, rules, start);
   }
 
-  parser(const char *s, size_t n) : parser(s, n, Rules()) {}
+  parser(const char *s, size_t n, std::string_view start = {})
+      : parser(s, n, Rules(), start) {}
 
-  parser(std::string_view sv, const Rules &rules)
-      : parser(sv.data(), sv.size(), rules) {}
+  parser(std::string_view sv, const Rules &rules, std::string_view start = {})
+      : parser(sv.data(), sv.size(), rules, start) {}
 
-  parser(std::string_view sv) : parser(sv.data(), sv.size(), Rules()) {}
+  parser(std::string_view sv, std::string_view start = {})
+      : parser(sv.data(), sv.size(), Rules(), start) {}
 
 #if defined(__cpp_lib_char8_t)
-  parser(std::u8string_view sv, const Rules &rules)
-      : parser(reinterpret_cast<const char *>(sv.data()), sv.size(), rules) {}
+  parser(std::u8string_view sv, const Rules &rules, std::string_view start = {})
+      : parser(reinterpret_cast<const char *>(sv.data()), sv.size(), rules,
+               start) {}
 
-  parser(std::u8string_view sv)
-      : parser(reinterpret_cast<const char *>(sv.data()), sv.size(), Rules()) {}
+  parser(std::u8string_view sv, std::string_view start = {})
+      : parser(reinterpret_cast<const char *>(sv.data()), sv.size(), Rules(),
+               start) {}
 #endif
 
   operator bool() { return grammar_ != nullptr; }
 
-  bool load_grammar(const char *s, size_t n, const Rules &rules) {
-    grammar_ = ParserGenerator::parse(s, n, rules, start_,
-                                      enablePackratParsing_, log_);
+  bool load_grammar(const char *s, size_t n, const Rules &rules,
+                    std::string_view start = {}) {
+    auto cxt = ParserGenerator::parse(s, n, rules, log_, start);
+    grammar_ = cxt.grammar;
+    start_ = cxt.start;
+    enablePackratParsing_ = cxt.enablePackratParsing;
     return grammar_ != nullptr;
   }
 
-  bool load_grammar(const char *s, size_t n) {
-    return load_grammar(s, n, Rules());
+  bool load_grammar(const char *s, size_t n, std::string_view start = {}) {
+    return load_grammar(s, n, Rules(), start);
   }
 
-  bool load_grammar(std::string_view sv, const Rules &rules) {
-    return load_grammar(sv.data(), sv.size(), rules);
+  bool load_grammar(std::string_view sv, const Rules &rules,
+                    std::string_view start = {}) {
+    return load_grammar(sv.data(), sv.size(), rules, start);
   }
 
-  bool load_grammar(std::string_view sv) {
-    return load_grammar(sv.data(), sv.size());
+  bool load_grammar(std::string_view sv, std::string_view start = {}) {
+    return load_grammar(sv.data(), sv.size(), start);
   }
 
   bool parse_n(const char *s, size_t n, const char *path = nullptr) const {
@@ -4671,7 +4688,7 @@ class parser {
   void enable_packrat_parsing() {
     if (grammar_ != nullptr) {
       auto &rule = (*grammar_)[start_];
-      rule.enablePackratParsing = enablePackratParsing_ && true;
+      rule.enablePackratParsing = enablePackratParsing_;
     }
   }
 
diff --git a/test/test1.cc b/test/test1.cc
index f63ad96..9816e85 100644
--- a/test/test1.cc
+++ b/test/test1.cc
@@ -423,7 +423,9 @@ TEST(GeneralTest, Skip_token_test2) {
 }
 
 TEST(GeneralTest, Custom_AST_test) {
-  struct CustomType { bool dummy = false; };
+  struct CustomType {
+    bool dummy = false;
+  };
   using CustomAst = AstBase<CustomType>;
 
   parser parser(R"(
@@ -646,11 +648,8 @@ TEST(GeneralTest, Calculator_test2) {
         NUMBER           <-  [0-9]+
     )";
 
-  std::string start;
-  bool enablePackratParsing = false;
-  auto grammar = ParserGenerator::parse(syntax, strlen(syntax), start,
-                                        enablePackratParsing, nullptr);
-  auto &g = *grammar;
+  auto cxt = ParserGenerator::parse(syntax, strlen(syntax), {}, nullptr, {});
+  auto &g = *cxt.grammar;
 
   // Setup actions
   auto reduce = [](const SemanticValues &vs) -> long {
@@ -679,7 +678,7 @@ TEST(GeneralTest, Calculator_test2) {
 
   // Parse
   long val;
-  auto r = g[start].parse_and_get_value("1+2*3*(4-5+6)/7-8", val);
+  auto r = g[cxt.start].parse_and_get_value("1+2*3*(4-5+6)/7-8", val);
 
   EXPECT_TRUE(r.ret);
   EXPECT_EQ(-3, val);
@@ -1285,3 +1284,40 @@ TEST(GeneralTest, PassingContextAndOutputParameter) {
   parser.parse<int>("42", dt, output);
   EXPECT_EQ(42, output);
 }
+
+TEST(GeneralTest, SpecifyStartRule) {
+  auto grammar = R"(
+    Start       <- A
+    A           <- B (',' B)*
+    B           <- '[one]' / '[two]'
+    %whitespace <- [ \t\n]*
+  )";
+
+  {
+    parser peg(grammar, "AAA");
+    EXPECT_FALSE(peg);
+  }
+
+  {
+    parser peg(grammar, "A");
+    EXPECT_TRUE(peg.parse(" [one] , [two] "));
+  }
+
+  {
+    parser peg(grammar);
+    EXPECT_TRUE(peg.parse(" [one] , [two] "));
+
+    peg.load_grammar(grammar, "A");
+    EXPECT_TRUE(peg.parse(" [one] , [two] "));
+  }
+
+  {
+    parser peg;
+
+    peg.load_grammar(grammar);
+    EXPECT_TRUE(peg.parse(" [one] , [two] "));
+
+    peg.load_grammar(grammar, "A");
+    EXPECT_TRUE(peg.parse(" [one] , [two] "));
+  }
+}