<form id="form1" runat="server">
  <div>
  <asp:GridView ID="gvClientList" AutoGenerateColumns="false" CellPadding="5" runat="server">
  <Columns>
  <asp:BoundField HeaderText="Client Name" DataField="_ClientName" />
  <asp:BoundField HeaderText="Client Phone" DataField="_ClientPhone" />
  <asp:BoundField HeaderText="Adress " DataField="_ClientAddress" />
  <asp:BoundField HeaderText="Due Payment" DataField="_DuePayment" />
  </Columns>
  <HeaderStyle BackColor="#6699FF" Font-Bold="true" ForeColor="White" />
  <FooterStyle BackColor="#3366CC" />
  </asp:GridView>
  </div>
  <br />
  <asp:Button ID="btnExportotExcelformat" runat="server" Text="Export data to Excel"
  OnClick="btnExportotExcelformat_Click" BackColor="#99CC00"
  BorderStyle="Solid" BorderWidth="2px" Font-Bold="True" ForeColor="#666633"
  Height="30px" />
  <br />
  </form>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.IO;
using System.Drawing;

protected void Page_Load(object sender, EventArgs e)
  {
  if (!IsPostBack)
  {
  AllClientList();
  }
  }
 
  /* Confirms that an HtmlForm control is rendered for the specified ASP.NET
  server control at run time. */
  public override void VerifyRenderingInServerForm(Control control)
  {
 
  }
 
  private void ChangHeadrowBackColor()
  {
 
  gvClientList.HeaderRow.Style.Add("background-color", "#FFFFFF");
 
 
  }
  private void DesingGridViewCell()
  {
  //After counting the headerroewcell of the gridview design exported Headercell
  for (int i = 0; i < gvClientList.HeaderRow.Cells.Count; i++)
  {
  gvClientList.HeaderRow.Cells[i].Style.Add("background-color","red");
 
 
  }
 
  }
 
  private void DesingGridViewColmun()
  {
 
  //After counting the cloums of the gridview design exported column
  for (int i = 0; i < gvClientList.Columns.Count; i++)
  {
  gvClientList.Columns[i].ItemStyle.BackColor = Color.AntiqueWhite;
  }
 
  }
 
  //Create temporarydata but real time data will be loaded form database
  private void AllClientList()
  {
 
  List<Client> ClientList = new List<Client>();
  ClientList.Add(new Client("Iqbal Hossain","01916751804","Noakhali",50000));
  ClientList.Add(new Client("Jabed Hossain", "01916751805", "Lakmipur", 70000));
  ClientList.Add(new Client("Fazlul Goni Mozumder", "01916751806", "Chandpur", 80000));
  ClientList.Add(new Client("Prince Hossain", "01916751807", "Rangpur", 90000));
  ClientList.Add(new Client("Mr Khorsed Alam", "01916751864", "Narayangonj", 50000));
  ClientList.Add(new Client("Sumaya Akter", "01916751804", "Sirajgonj", 50000));
  ClientList.Add(new Client("Siful Islam", "01916751808", "Dhaka", 1000000));
  ClientList.Add(new Client("Mizanur Rahman", "01916751807", "Dhaka", 20000));
  gvClientList.DataSource = ClientList;
  gvClientList.DataBind();
 
 
 
 
 
  }
 
  protected void btnExportoWordFormat_Click(object sender, EventArgs e)
  {
  Response.ClearContent();
  Response.Buffer = true;
  Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", "ClientList.doc"));
  Response.ContentType = "application/ms-word";
  StringWriter sw = new StringWriter();
  HtmlTextWriter htw = new HtmlTextWriter(sw);
  AllClientList();
  //Grid View Header row backcolor styling function
  ChangHeadrowBackColor();
  //Grid View Cell styling function
  DesingGridViewCell();
  //Grid View Column styling function
  DesingGridViewColmun();
  gvClientList.RenderControl(htw);
  Response.Write(sw.ToString());
 
  Response.End();
  }
 
 
 
  //Create class for temporarydata
  public class Client
  {
 
 
  public Client(string ClientName, string ClientPhone, string ClientAddress, decimal DuePayment)
  {
 
  _ClientName = ClientName;
  _ClientPhone = ClientPhone;
  _ClientAddress = ClientAddress;
  _DuePayment = DuePayment;
 
  }
 
  public string _ClientName { get; set; }
 
  public string _ClientPhone { get; set; }
 
  public string _ClientAddress { get; set; }
 
  public decimal _DuePayment { get; set; }

Output ms-word format