I Found the solution for this. Now it works fine.
Response.Clear();
Response.ClearContent();
Response.ClearHeaders();
Response.Charset = "utf-8";
Response.AddHeader("content-disposition", "attachment; filename=FileName.xls");
Response.ContentEncoding = System.Text.Encoding.Unicode;
Response.ContentType = "application/ms-excel";
Response.BinaryWrite(System.Text.Encoding.Unicode.GetPreamble());
StringWriter sw = new StringWriter();
HtmlTextWriter hw = new HtmlTextWriter(sw);
comments.RenderControl(hw);
Response.Output.Write(sw.ToString());
Response.Flush();
Response.End();
Thank you.