i have done minor change and now things are working fine.
Use Response.End(); instead of 
HttpContext.Current.ApplicationInstance.CompleteRequest();
protected void btnExport_click(object sender, EventArgs e)
  {
      try
      {
 
          Response.Clear();
          Response.AddHeader("content-disposition", "attachment;filename=iQuiz.xls");
          Response.Charset = "";
          Response.ContentType = "application/vnd.xls";
          System.IO.StringWriter stringWrite = new System.IO.StringWriter();
          System.Web.UI.HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);
          GridView1.RenderControl(htmlWrite);
          Response.Write(stringWrite.ToString());
          Response.End();
         // HttpContext.Current.ApplicationInstance.CompleteRequest();
      }
 
      catch (Exception ex)
      { }
 
      finally
      { }
  }