<script type="text/javascript">
function ShowHideDiv(chkPassport) {
var dvPassport = document.getElementById("dvPassport");
dvPassport.style.display = chkPassport.checked ? "block" : "none";
}
</script>
<label for="chkPassport">
<asp:CheckBox ID="chkPassport" Text="Do you have Passport?" runat="server" onclick="ShowHideDiv(this)" />
</label>
<hr />
<div id="dvPassport" style="display: none">
Passport Number:
<asp:TextBox ID="txtPassPort" runat="server" />
</div>