Below code resolved my problem.
StringWriter sw = new StringWriter();
HtmlTextWriter hw = new HtmlTextWriter(sw);
PlaceHolder1.DataBind();
PlaceHolder1.RenderControl(hw);
string strPath = Server.MapPath("~/Files/Test.doc");
StreamWriter sWriter = new StreamWriter(strPath);
sWriter.Write(sw.ToString());
sWriter.Close();
May be it's due to the placeholder i used in Aspx file.
<asp:PlaceHolder ID="PlaceHolder1" runat="server" />
I was actually using StringBuilder and all of the my logic is in code behind.