diff --git a/lib/fluent/plugin/out_s3.rb b/lib/fluent/plugin/out_s3.rb
index 8b173c70..dc546a9e 100644
--- a/lib/fluent/plugin/out_s3.rb
+++ b/lib/fluent/plugin/out_s3.rb
@@ -209,6 +209,8 @@ def configure(conf)
         end
       end
 
+      check_s3_path_safety(conf)
+
       # For backward compatibility
       # TODO: Remove time_slice_format when end of support compat_parameters
       @configured_time_slice_format = conf['time_slice_format']
@@ -451,6 +453,16 @@ def process_s3_object_key_format
       }
     end
 
+    def check_s3_path_safety(conf)
+      unless conf.has_key?('s3_object_key_format')
+        log.warn "The default value of s3_object_key_format will use ${chunk_id} instead of %{index} to avoid object conflict in v2"
+      end
+
+      if (@buffer_config.flush_thread_count > 1) && ['${chunk_id}', '%{uuid_flush}'].none? { |key| @s3_object_key_format.include?(key) }
+        log.warn "No ${chunk_id} or %{uuid_flush} in s3_object_key_format with multiple flush threads. Recommend to set ${chunk_id} or %{uuid_flush} to avoid data lost by object conflict"
+      end
+    end
+
     def check_apikeys
       @bucket.objects(prefix: @path, :max_keys => 1).first
     rescue Aws::S3::Errors::NoSuchBucket