diff --git a/mysql-test/lib/mtr_report.pm b/mysql-test/lib/mtr_report.pm
index 7d944ade71a62..97c48c1911205 100644
--- a/mysql-test/lib/mtr_report.pm
+++ b/mysql-test/lib/mtr_report.pm
@@ -76,6 +76,30 @@ if (-t STDOUT) {
   }
 }
 
+# On Windows, stdio does not support line buffering
+# This can make MTR output from multiple forked processes interleaved, messed up.
+# Below is DYI stdout line buffering.
+my $out_line="";
+
+# Flush buffered line
+sub flush_out {
+    print $out_line;
+    $out_line = "";
+}
+
+# Print to stdout
+sub print_out {
+  if(IS_WIN32PERL) {
+    $out_line .= $_[0];
+    # Flush buffered output on new lines.
+    if (rindex($_[0], "\n") != -1) {
+      flush_out();
+    }
+  } else {
+    print($_[0]);
+  }
+}
+
 sub titlebar_stat($) {
 
   sub time_format($) {
@@ -116,10 +140,10 @@ sub _mtr_report_test_name ($) {
 
   return unless defined $verbose;
 
-  print _name(). _timestamp();
-  printf "%-40s ", $tname;
+  print_out _name(). _timestamp();
+  print_out (sprintf "%-40s ", $tname);
   my $worker = $tinfo->{worker};
-  print "w$worker " if defined $worker;
+  print_out "w$worker " if defined $worker;
 
   return $tname;
 }
@@ -661,14 +685,14 @@ sub mtr_report (@) {
   {
     my @s = split /\[ (\S+) \]/, _name() . "@_\n";
     if (@s > 1) {
-      print $s[0];
+      print_out $s[0];
       &$set_color($s[1]);
-      print "[ $s[1] ]";
+      print_out "[ $s[1] ]";
       &$set_color('reset');
-      print $s[2];
+      print_out $s[2];
       titlebar_stat($s[1]) if $set_titlebar;
     } else {
-      print $s[0];
+      print_out $s[0];
     }
   }
 }
@@ -676,6 +700,7 @@ sub mtr_report (@) {
 
 # Print warning to screen
 sub mtr_warning (@) {
+  flush_out();
   print STDERR _name(). _timestamp().
     "mysql-test-run: WARNING: ". join(" ", @_). "\n";
 }
@@ -683,7 +708,7 @@ sub mtr_warning (@) {
 
 # Print error to screen and then exit
 sub mtr_error (@) {
-  IO::Handle::flush(\*STDOUT) if IS_WINDOWS;
+  flush_out();
   print STDERR _name(). _timestamp().
     "mysql-test-run: *** ERROR: ". join(" ", @_). "\n";
   if (IS_WINDOWS)