private void ExportGridView()
    {
        string attachment = "attachment; filename=fileName.xls";
        Response.ClearContent();
        Response.AddHeader("content-disposition", attachment);
        Response.ContentType = "application/ms-excel";
        //Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
        StringWriter sw = new StringWriter();
        HtmlTextWriter htw = new HtmlTextWriter(sw);
        // Create a form to contain the grid
        HtmlForm frm = new HtmlForm();
        //grd_InitiatedDayCase.Parent.Controls.Add(frm);
        innerData.Parent.Controls.Add(frm);
        frm.Attributes["runat"] = "server";
        //frm.Controls.Add(grd_InitiatedDayCase);
        frm.Controls.Add(innerData);
        frm.RenderControl(htw);
        //GridView1.RenderControl(htw);
        Response.Write(sw.ToString());
        Response.End();
    }

 protected void btn_ExportToExcel_Click(object sender, EventArgs e)
    {
        ExportGridView();
    }

< asp:LinkButton ID="btn_ExportToExcel" runat="server" OnClick="btn_ExportToExcel_Click" > Export To Excel < /asp:LinkButton >

< div id="innerData" runat="server" >
  put the control here whose data you want to show
< /div >

using System.IO;