Response.Clear();
        Response.Buffer = true;
        Response.Charset = "GB2312";
        Response.AppendHeader("Content-Disposition", "attachment;filename=out.xls");
        Response.ContentType = "application/ms-excel";
        this.EnableViewState = false;
        System.IO.StringWriter swOut = new System.IO.StringWriter();
        HtmlTextWriter hTw = new HtmlTextWriter(swOut);
        GridView1.RenderControl(hTw);
        Response.Write(swOut.ToString());
        Response.End();

------解决方案--------------------

打开excel赋值给单元格，再使用winrar等压缩输出
string style = @"<style> .text { mso-number-format:\@; } </script> ";  
Response.ClearContent();
Response.AddHeader("content-disposition", "attachment; filename=MyExcelFile.xls");
Response.ContentType = "application/excel";
StringWriter sw = new StringWriter();
HtmlTextWriter htw = new HtmlTextWriter(sw);
gv.RenderControl(htw);
Response.Write(style);  
Response.Write(sw.ToString());
Response.End();
public override void VerifyRenderingInServerForm(Control control)
{
}


------解决方案--------------------
http://www.csharp360.com/bbs/viewthread.php?tid=142&extra=page%3D1
上面有