diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -4072,12 +4072,16 @@
         }
 
         CValidationState state;
-        // Ensure that CheckBlock() passes before calling AcceptBlock, as
-        // belt-and-suspenders.
-        bool ret = CheckBlock(config, *pblock, state);
 
+        // CheckBlock() does not support multi-threaded block validation
+        // because CBlock::fChecked can cause data race.
+        // Therefore, the following critical section must include the
+        // CheckBlock() call as well.
         LOCK(cs_main);
 
+        // Ensure that CheckBlock() passes before calling AcceptBlock, as
+        // belt-and-suspenders.
+        bool ret = CheckBlock(config, *pblock, state);
         if (ret) {
             // Store to disk
             ret = g_chainstate.AcceptBlock(