I used the below line to open ShowModalDialog of javascript from asp.net code behind.
Page page = HttpContext.Current.CurrentHandler as Page;
ScriptManager.RegisterStartupScript(page, page.GetType(), "OpenModalDialog", "<script type=text/javascript>window.showModalDialog('ClockPopUP.aspx', null, 'dialogWidth:290px;dialogHeight:270px;status:no'); </script>", false);