 <p>This works for me. </p> <p><strong>ASPX :</strong> </p> <pre><code>&lt;iframe id="ContentIframe" runat="server"&gt;&lt;/iframe&gt; </code></pre> <p>I can access the iframe directly via id.</p> <p><strong>Code Behind :</strong> </p> <pre><code>ContentIframe.Attributes["src"] = "stackoverflow.com"; </code></pre>