diff --git a/src/core/thread/mesh_forwarder.cpp b/src/core/thread/mesh_forwarder.cpp
index ee749fe92..9574bbd54 100644
--- a/src/core/thread/mesh_forwarder.cpp
+++ b/src/core/thread/mesh_forwarder.cpp
@@ -770,11 +770,11 @@ start:
         payload += fragmentHeaderLength;
         headerLength += fragmentHeaderLength;
 
-        fragmentLength = (aFrame.GetMaxPayloadLength() - headerLength) & ~0x7;
+        fragmentLength = aFrame.GetMaxPayloadLength() - headerLength;
 
         if (payloadLength > fragmentLength)
         {
-            payloadLength = fragmentLength;
+            payloadLength = (fragmentLength & ~0x7);
         }
 
         // Copy IPv6 Payload