diff --git a/src/Verifier.sol b/src/Verifier.sol
index ac1b75b..309e999 100644
--- a/src/Verifier.sol
+++ b/src/Verifier.sol
@@ -30,10 +30,7 @@ abstract contract Verifier is IVerifier {
         bytes32[32] messageProof;
     }
 
-    /// @notice Fetch message root oracle.
-    /// @param chainId The destination chain id.
-    /// @param blockNumber The block number where the message root is located.
-    /// @return Message root in destination chain.
+    /// @inheritdoc IVerifier
     function merkleRoot(uint256 chainId, uint256 blockNumber) public view virtual returns (bytes32);
 
     /// @inheritdoc IVerifier
diff --git a/src/interfaces/IVerifier.sol b/src/interfaces/IVerifier.sol
index 5a6dbd7..5b25f7b 100644
--- a/src/interfaces/IVerifier.sol
+++ b/src/interfaces/IVerifier.sol
@@ -18,6 +18,12 @@
 pragma solidity 0.8.17;
 
 interface IVerifier {
+    /// @notice Fetch message root oracle.
+    /// @param chainId The destination chain id.
+    /// @param blockNumber The block number where the message root is located.
+    /// @return Message root in destination chain.
+    function merkleRoot(uint256 chainId, uint256 blockNumber) external view returns (bytes32);
+
     /// @notice Verify message proof
     /// @dev Message proof provided by relayer. Oracle should provide message root of
     ///      source chain, and verify the merkle proof of the message hash.