diff --git a/src/main/java/org/hydev/clock_api/controller/UserController.java b/src/main/java/org/hydev/clock_api/controller/UserController.java
index 27d9e8c..ce1b6cb 100644
--- a/src/main/java/org/hydev/clock_api/controller/UserController.java
+++ b/src/main/java/org/hydev/clock_api/controller/UserController.java
@@ -13,7 +13,6 @@ import org.springframework.web.bind.annotation.RequestHeader;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RestController;
 
-import javax.validation.constraints.NotNull;
 import javax.validation.constraints.Pattern;
 import java.util.List;
 
@@ -32,7 +31,11 @@ public class UserController {
     // https://www.baeldung.com/spring-rest-http-headers
     // TODO: This method should be synchronized to avoid race condition.
     // Also, this method should not be private, or else cannot use userRepository.
-    public synchronized ResponseEntity<String> register(
+
+    // TODO: 2021/1/22 Need a better design! 
+    // Controller Return error code list as List<String>, or return uuid as String.
+    @SuppressWarnings("rawtypes")
+    public synchronized ResponseEntity register(
             // username & password shouldn't be null, and should match thr regex.
             // [!] @RequestHeader(required = false) makes no need make another error handler.
             // [!] And also, ExceptionHandler of MissingRequestHeaderException cannot deal with all missing fields.
@@ -42,9 +45,10 @@ public class UserController {
             @RequestHeader String password
     ) {
         // First, spring will check args. If not pass there regex, raise ConstraintViolationException.
-        // Check if username not exists.
+        // Then, we check if username not exists. If username already exists, return ErrorCode with 409 conflict.
         if (userRepository.existsByUsername(username))
-            return ResponseEntity.status(HttpStatus.NOT_ACCEPTABLE).body(ErrorCode.USER_NAME_ALREADY_EXISTS);
+            // TODO: 2021/1/22 Using library instead of hand make.
+            return ResponseEntity.status(HttpStatus.CONFLICT).body(List.of(ErrorCode.USER_NAME_ALREADY_EXISTS));
 
         User user = new User();
         user.setUsername(username);
diff --git a/src/test/java/org/hydev/clock_api/UserRegisterDeleteNodeTest.kt b/src/test/java/org/hydev/clock_api/UserRegisterDeleteNodeTest.kt
index 05091e3..15e3f6b 100644
--- a/src/test/java/org/hydev/clock_api/UserRegisterDeleteNodeTest.kt
+++ b/src/test/java/org/hydev/clock_api/UserRegisterDeleteNodeTest.kt
@@ -122,7 +122,7 @@ class UserRegisterDeleteNodeTest {
                 // Notice: inserted user should be delete.
                 val tempUuid = result.response.contentAsString
                 mockMvc.perform(post(REGISTER_NODE).header(H_USERNAME, V_USERNAME).header(H_PASSWORD, V_PASSWORD))
-                    .andExpect(status().isNotAcceptable)
+                    .andExpect(status().isConflict)
                     .andExpect(content().json(String.format("[\"%s\"]", USER_NAME_ALREADY_EXISTS)))
                 userRepository.deleteById(tempUuid)
             }