Grid Export Format Problem

When data comes more than 30 Rows in Grid then the Excel goes in different format.There is parsian/Urdu Font in Grid data.

<div id="PrintDiv">
    <asp:GridView ID="gvExamReport" ClientIDMode="Static" runat="server" OnDataBound="gvExamReport_DataBound"
        HeaderStyle-Height="20" HeaderStyle-HorizontalAlign="Center" OnRowDataBound="gvExamReport_RowDataBound">
        <Columns>
            <asp:TemplateField HeaderText="مرتبة">
                <ItemTemplate>
                    <%#Container.DataItemIndex+1 %>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
        <HeaderStyle BackColor="#a6f0c0" Height="35" Font-Size="Medium" />
        <RowStyle HorizontalAlign="Center" />
    </asp:GridView>
    <br />
    <asp:Button ID="Button1" runat="server" Text="Import To Excel" CssClass="btn btn-primary"
        OnClick="Button1_Click" />
</div>

try
{
    Response.ClearContent();
    Response.Buffer = true;
    Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", "Customers.xls"));
    Response.ContentType = "application/ms-excel";
    StringWriter sw = new StringWriter();
    HtmlTextWriter htw = new HtmlTextWriter(sw);
    gvExamReport.AllowPaging = false;
    gvExamReport.DataSource = ViewState["datag"];
    gvExamReport.DataBind();
    gvExamReport.HeaderRow.Style.Add("background-color", "#FFFFFF");
    gvExamReport.RenderControl(htw);
    Response.Write(sw.ToString());
    Response.Flush();
    Response.End();
}