protected void btn_ToExcel_Click(object sender, EventArgs e)

    {

        Response.ClearContent();

        Response.AddHeader("content-disposition", "attachment; filename=MyExcelFile.xls");

        Response.ContentType = "application/excel";

        Response.Write(GetDataTableHtml((DataTable)ViewState["tt"]));

        Response.End();

    }

private string GetDataTableHtml(DataTable dt)

    {

        StringBuilder sb = new StringBuilder();

        sb.Append("<table cellspacing=\"0\" rules=\"all\" border=\"1\" style=\"border-collapse:collapse;\">");

        sb.Append("<tr>");

        foreach (DataColumn column in dt.Columns)

        {

            sb.AppendFormat("<td>{0}</td>", column.Caption);

        }

        sb.Append("</tr>");

        foreach (DataRow row in dt.Rows)

        {

            sb.Append("<tr>");

            for (int i = 0; i < dt.Columns.Count;i++ )

            {

                string html = row[i].ToString();

                sb.AppendFormat("<td>{0}</td>", html);

            }

            sb.Append("</tr>");

        }

        sb.Append("</table>");

        return sb.ToString();

    }