diff --git a/contracts/NFTPool/impl/FilterAdmin.sol b/contracts/NFTPool/impl/FilterAdmin.sol
index 927b48e..359167b 100644
--- a/contracts/NFTPool/impl/FilterAdmin.sol
+++ b/contracts/NFTPool/impl/FilterAdmin.sol
@@ -73,6 +73,7 @@ contract FilterAdmin is InitializableInternalMintableERC20 {
 
     //================ View ================
     function queryMintFee(uint256 rawAmount)
+        view
         public
         returns (
             uint256 poolFee,
@@ -87,6 +88,7 @@ contract FilterAdmin is InitializableInternalMintableERC20 {
     }
 
     function queryBurnFee(uint256 rawAmount)
+        view
         public
         returns (
             uint256 poolFee,
diff --git a/contracts/NFTPool/intf/IFilterAdmin.sol b/contracts/NFTPool/intf/IFilterAdmin.sol
index 8cd63b7..29e68d5 100644
--- a/contracts/NFTPool/intf/IFilterAdmin.sol
+++ b/contracts/NFTPool/intf/IFilterAdmin.sol
@@ -8,9 +8,9 @@
 pragma solidity 0.6.9;
 
 interface IFilterAdmin {
-    function _OWNER_() external returns (address);
+    function _OWNER_() external view returns (address);
 
-    function _CONTROLLER_() external returns (address);
+    function _CONTROLLER_() external view returns (address);
 
     function init(
         address owner,