diff --git a/specs/opcode/38CODESIZE.md b/specs/opcode/38CODESIZE.md
new file mode 100644
index 000000000..146932262
--- /dev/null
+++ b/specs/opcode/38CODESIZE.md
@@ -0,0 +1,35 @@
+# CODESIZE opcode
+
+## Procedure
+
+The `CODESIZE` opcode gets the size of code running in current environment.
+
+## EVM behaviour
+
+The `CODESIZE` opcode pushes the size of code running in the current environment to the top of the stack.
+
+## Circuit behaviour
+
+1. Lookup the code size from the bytecode table
+2. Do busmapping lookup for stack write operation
+
+## Constraints
+
+1. opId = 0x38
+2. State transition:
+   - gc + 1 (1 stack write)
+   - stack_pointer - 1
+   - pc + 1
+   - gas + 2
+3. Lookups: 2
+   - `codesize` (bytecode_length) from the bytecode table
+   - `codesize` is on top of stack
+
+## Exceptions
+
+1. stack overflow: stack is full, stack pointer = 0
+2. out of gas: remaining gas is not enough
+
+## Code
+
+Please refer [`codesize`](src/zkevm_specs/evm/execution/codesize.py).
diff --git a/src/zkevm_specs/evm/execution/__init__.py b/src/zkevm_specs/evm/execution/__init__.py
index 287dc13a2..7a22349ce 100644
--- a/src/zkevm_specs/evm/execution/__init__.py
+++ b/src/zkevm_specs/evm/execution/__init__.py
@@ -19,6 +19,7 @@
 from .calldatacopy import *
 from .calldataload import *
 from .codecopy import *
+from .codesize import *
 from .gas import *
 from .iszero import *
 from .jump import *
@@ -50,6 +51,7 @@
     ExecutionState.CALLDATALOAD: calldataload,
     ExecutionState.CALLDATASIZE: calldatasize,
     ExecutionState.CODECOPY: codecopy,
+    ExecutionState.CODESIZE: codesize,
     ExecutionState.BlockCtx: blockctx,
     ExecutionState.JUMP: jump,
     ExecutionState.JUMPI: jumpi,
diff --git a/src/zkevm_specs/evm/execution/codesize.py b/src/zkevm_specs/evm/execution/codesize.py
new file mode 100644
index 000000000..93cc69bcb
--- /dev/null
+++ b/src/zkevm_specs/evm/execution/codesize.py
@@ -0,0 +1,22 @@
+from ...util import N_BYTES_U64
+from ..instruction import Instruction, Transition
+from ..opcode import Opcode
+
+
+def codesize(instruction: Instruction):
+    opcode = instruction.opcode_lookup(True)
+    instruction.constrain_equal(opcode, Opcode.CODESIZE)
+
+    code_size = instruction.bytecode_length(instruction.curr.code_hash)
+
+    instruction.constrain_equal(
+        instruction.rlc_to_fq(instruction.stack_push(), N_BYTES_U64),
+        code_size.expr(),
+    )
+
+    instruction.step_state_transition_in_same_context(
+        opcode,
+        rw_counter=Transition.delta(1),
+        program_counter=Transition.delta(1),
+        stack_pointer=Transition.delta(-1),
+    )
diff --git a/tests/evm/test_codesize.py b/tests/evm/test_codesize.py
new file mode 100644
index 000000000..1fd5fe1b2
--- /dev/null
+++ b/tests/evm/test_codesize.py
@@ -0,0 +1,55 @@
+import pytest
+
+from zkevm_specs.evm import (
+    Bytecode,
+    ExecutionState,
+    RWDictionary,
+    StepState,
+    Tables,
+    verify_steps,
+)
+from zkevm_specs.util import rand_fq, RLC
+
+
+def test_codesize():
+    randomness = rand_fq()
+
+    bytecode = Bytecode().codesize().stop()
+    codesize = len(bytecode.code)
+    bytecode_hash = RLC(bytecode.hash(), randomness)
+
+    tables = Tables(
+        block_table=set(),
+        tx_table=set(),
+        bytecode_table=set(bytecode.table_assignments(randomness)),
+        rw_table=set(RWDictionary(9).stack_write(1, 1023, RLC(codesize, randomness)).rws),
+    )
+
+    verify_steps(
+        randomness=randomness,
+        tables=tables,
+        steps=[
+            StepState(
+                execution_state=ExecutionState.CODESIZE,
+                rw_counter=9,
+                call_id=1,
+                is_root=True,
+                is_create=False,
+                code_hash=bytecode_hash,
+                program_counter=0,
+                stack_pointer=1024,
+                gas_left=2,
+            ),
+            StepState(
+                execution_state=ExecutionState.STOP,
+                rw_counter=10,
+                call_id=1,
+                is_root=True,
+                is_create=False,
+                code_hash=bytecode_hash,
+                program_counter=1,
+                stack_pointer=1023,
+                gas_left=0,
+            ),
+        ],
+    )