diff --git a/src/calibre/ebooks/docx/char_styles.py b/src/calibre/ebooks/docx/char_styles.py
index 8b45b2c211..a9d2a43cdb 100644
--- a/src/calibre/ebooks/docx/char_styles.py
+++ b/src/calibre/ebooks/docx/char_styles.py
@@ -209,6 +209,9 @@ class RunStyle(object):
 
             if self.highlight is not inherit and self.highlight != 'transparent':
                 c['background-color'] = self.highlight
+
+            if self.b:
+                c['font-weight'] = 'bold'
         return self._css
 
     def same_border(self, other):