try like this
Response.Clear();
Response.AddHeader("content-disposition", "attachment;filename=NAVEEN.xls");
Response.Charset = "";
Response.ContentType = "application/vnd.xls";
System.IO.StringWriter stringWrite = new System.IO.StringWriter();
System.Web.UI.HtmlTextWriter htmlWrite = new HtmlTextWriter(stringWrite);
GridView1.RenderControl(htmlWrite);
Response.Write("<meta http-equiv=\"Content-Type\" content=\"text/html; charset=utf-8\" />");
Response.Write(stringWrite.ToString());
Response.End();
or
see this link similar discussion
http://forums.asp.net/t/1639973.aspx[
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