 <p>You should use <code>$("#&lt;%=UploadButton.ClientID%&gt;")</code> instead of <code>$('UploadButton')</code> because <code>asp:Button</code> elements generates not in element with just id <code>UploadButton</code></p> <pre><code>function ConfirmSendData() { var r = confirm("Êtes vous bien: " + document.getElementById("&lt;%=DDL_LaveurLA.ClientID%&gt;").options[document.getElementById("&lt;%=DDL_LaveurLA.ClientID%&gt;").selectedIndex].text + " sinon veuillez changer dans le champ spécifié 'Laveur'"); if (r == true) { var clickButton = document.getElementById("&lt;%= UploadButton.ClientID %&gt;"); clickButton.click(); // alternative variant for jquery // $("#&lt;%=UploadButton.ClientID%&gt;").click(); } } </code></pre> <p>also another thing that function <code>ConfirmSendData</code> needs to <code>return false</code> to prevent the submitting data by first button</p>