<asp:Button ID="Btn1" runat="server" Text="SELECT" onclick="Btn1_Click1"/>
  <asp:Button ID="ExportTo" runat="server" Text="Экспорт" onclick="ExportTo_Click" />
  <asp:GridView ID="GV1" runat="server"></asp:GridView>

  DataSet ds = new DataSet();

 protected void Btn1_Click1(object sender, EventArgs e)
        {
            string sql = null;
            connec.Open();
            sql = "SELECT * FROM TEST1";
            OracleDataAdapter adap = new OracleDataAdapter(sql, connec);

            adap.Fill(ds);

            GV1.DataSource = ds.Tables[0];
            GV1.DataBind();
        }

 protected void ExportTo_Click(object sender, EventArgs e)
        {
            ToExcel("report.xls", GV1);
        }

 public static void ToExcel(string fileName, GridView gv)
        {

            HttpContext.Current.Response.Clear();
            HttpContext.Current.Response.AddHeader(
                "content-disposition", string.Format("attachment; filename={0}", fileName));
            HttpContext.Current.Response.ContentType = "application/msexcel";

            using (StringWriter sw = new StringWriter())
            {
                using (HtmlTextWriter htw = new HtmlTextWriter(sw))
                {

                    GridViewWrapper(gv).RenderControl(htw);
                    HttpContext.Current.Response.Write(sw.ToString());
                    HttpContext.Current.Response.End();
                }
            }
        }

        private static Table GridViewWrapper(GridView gv)
        {
            Table table = new Table();
            table.GridLines = gv.GridLines;
            if (gv.HeaderRow != null)
            {
                WebForm2.PrepareControlForExport(gv.HeaderRow);
                table.Rows.Add(gv.HeaderRow);
            }
            foreach (GridViewRow row in gv.Rows)
            {
                WebForm2.PrepareControlForExport(row);
                table.Rows.Add(row);
            }

            if (gv.FooterRow != null)
            {
                WebForm2.PrepareControlForExport(gv.FooterRow);
                table.Rows.Add(gv.FooterRow);
            }
            return table;
        }

        private static void PrepareControlForExport(Control control)
        {
            for (int i = 0; i < control.Controls.Count; i++)
            {
                var current = control.Controls[i];
                if (current is LinkButton)
                {
                    control.Controls.Remove(current);
                    control.Controls.AddAt(i, new LiteralControl((current as LinkButton).Text));
                }
                else if (current is ImageButton)
                {
                    control.Controls.Remove(current);
                    control.Controls.AddAt(i, new LiteralControl((current as ImageButton).AlternateText));
                }
                else if (current is HyperLink)
                {
                    control.Controls.Remove(current);
                    control.Controls.AddAt(i, new LiteralControl((current as HyperLink).Text));
                }
                else if (current is DropDownList)
                {
                    control.Controls.Remove(current);
                    control.Controls.AddAt(i, new LiteralControl((current as DropDownList).SelectedItem.Text));
                }
                else if (current is CheckBox)
                {
                    control.Controls.Remove(current);
                    control.Controls.AddAt(i, new LiteralControl((current as CheckBox).Checked ? "True" : "False"));
                }
                if (current.HasControls())
                {
                    PrepareControlForExport(current);
                }
            }
        }