//on export Button click
protected void btn_xport_Click(object sender, EventArgs e)
    {
           //passing name for Excel and our gridview data 
        ExportToExcel("ExcelName", gridview);
    }

    public override void VerifyRenderingInServerForm(Control control)
    {

    }

    private void ExportToExcel(string strFileName, GridView gv_inventry)
    {
        Response.Clear();
        Response.Buffer = true;
        Response.ContentType = "application/vnd.ms-excel";
        Response.AddHeader("content-disposition","attachment;filename=" +  strFileName);
        Response.Charset = "";
        this.EnableViewState = false;
        System.IO.StringWriter sw = new StringWriter();
        System.Web.UI.HtmlTextWriter htw = new HtmlTextWriter(sw);
        gv_inventry.RenderControl(htw);
        Response.Write(sw.ToString());
        Response.End();
    }