use code below
<script> $(document).ready(function () { $("[id$='chkSendMail']").attr("onchange", "ShowMailSection()"); }
function ShowMailSection() {
$("[id$='chkSendMail']").attr("onchange", "ShowMailSection()"); } function ShowMailSection() { if ($("[id$='chkSendMail'][type='checkbox']:checked").length >0){ $("[id$='SecEmail']").removeClass("Hide"); } </script>