diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index a23663a48e..425a1e494c 100644
--- a/docs/formatdomain.html.in
+++ b/docs/formatdomain.html.in
@@ -477,13 +477,16 @@
   ...
   <devices>
     <disk type='file'>
-      <driver name="tap" type="aio" cache="default">
+      <driver name="tap" type="aio" cache="default"/>
       <source file='/var/lib/xen/images/fv0'/>
       <target dev='hda' bus='ide'/>
       <encryption type='...'>
         ...
       </encryption>
       <shareable/>
+      <serial>
+        ...
+      </serial>
     </disk>
   </devices>
   ...</pre>
@@ -532,6 +535,11 @@
           between domains (assuming the hypervisor and OS support this),
           which means that caching should be deactivated for that device.
       </dd>
+      <dt><code>serial</code></dt>
+      <dd>If present, this specify serial number of virtual hard drive.
+          For example, it may look as <code>&lt;serial&gt;WD-WMAP9A966149&lt;/serial&gt;</code>.
+          <span class="since">Since 0.7.1</span>
+      </dd>
     </dl>
 
     <h4><a name="elementsUSB">USB and PCI devices</a></h4>