protected void binddata()
    {
        con.ConnectionString = "data source=.;initial catalog=priya;integrated security=true";
        con.Open();
        com.Connection = con;
        com.CommandText = "select * from emp";
        ada.SelectCommand = com;
        ada.Fill(ds, "emp");
        GridView1.DataSource = ds;
        GridView1.DataBind();
                  }

In button Click Event:
   protected void Export_Click(object sender, EventArgs e)
{
    Response.Clear();
    Response.Buffer = true;
    //For Word:
    //Response.AddHeader("content-disposition", "attachment;filename=data.doc");
    //Response.ContentType = "application/vnd.ms-word";
    //For Excel
    Response.AddHeader("content-disposition", "attachment;filename=data.xls");
    Response.ContentType = "application/vnd.ms-excel";
    Response.Charset = "";
    this.EnableViewState = false;
    System.IO.StringWriter oStringWriter = new System.IO.StringWriter();
    System.Web.UI.HtmlTextWriter oHtmlTextWriter = new    System.Web.UI.HtmlTextWriter(oStringWriter);
     GridView1.RenderControl(oHtmlTextWriter);
    Response.Write(oStringWriter.ToString());
    Response.End(); 
    }


public override void VerifyRenderingInServerForm(Control control)
    {
        /* Confirms that an HtmlForm control is rendered for the specified ASP.NET
           server control at run time. */
    }