Hi sambath,
Refer the below code.
HTML
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <style type="text/css">
        .modal-body
        {
            height: 350px;
            overflow: hidden;
        }
        .modal-body:hover
        {
            overflow-y: auto;
        }
        .modal-open
        {
            overflow: scroll;
        }
    </style>
    <script type="text/javascript">
        function checkAll(objRef) {
            var GridView = objRef.parentNode.parentNode.parentNode;
            var inputList = GridView.getElementsByTagName("input");
            for (var i = 0; i < inputList.length; i++) {
                var row = inputList[i].parentNode.parentNode;
                if (inputList[i].type == "checkbox" && objRef != inputList[i]) {
                    if (objRef.checked) {
                        inputList[i].checked = true;
                    }
                    else {
                        inputList[i].checked = false;
                    }
                }
            }
        }
        function Check_Click(objRef) {
            var row = objRef.parentNode.parentNode;
            var GridView = row.parentNode;
            var inputList = GridView.getElementsByTagName("input");
            for (var i = 0; i < inputList.length; i++) {
                var headerCheckBox = inputList[0];
                var checked = true;
                if (inputList[i].type == "checkbox" && inputList[i] != headerCheckBox) {
                    if (!inputList[i].checked) {
                        checked = false;
                        break;
                    }
                }
            }
            headerCheckBox.checked = checked;
        }  
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:ScriptManager runat="server" />
        <asp:Panel ID="pnlsms" runat="server" Style="display: none;">
            <div class="modal-dialog">
                <div class="modal-content">
                    <div class="modal-header">
                        <h4 class="modal-title panel-title" id="ModalTitle">
                            Select Members</h4>
                    </div>
                    <div class="modal-body">
                        <asp:UpdatePanel ID="upModal" runat="server">
                            <ContentTemplate>
                                <center>
                                    <asp:GridView ID="gridsendsms" DataKeyNames="ID" AutoGenerateColumns="False" BorderWidth="5px"
                                        BorderColor="YellowGreen" BorderStyle="Groove" runat="server">
                                        <Columns>
                                            <asp:TemplateField>
                                                <HeaderTemplate>
                                                    Select All
                                                    <asp:CheckBox ID="cbSelectAll" runat="server" onclick="checkAll(this);" />
                                                </HeaderTemplate>
                                                <ItemTemplate>
                                                    <asp:CheckBox ID="chkSelect" runat="server" onclick="Check_Click(this)" />
                                                </ItemTemplate>
                                                <ItemStyle HorizontalAlign="Right" />
                                            </asp:TemplateField>
                                            <asp:TemplateField HeaderText="S.No" ItemStyle-HorizontalAlign="Center">
                                                <ItemTemplate>
                                                    <%#Container.DataItemIndex+1 %>
                                                </ItemTemplate>
                                                <ItemStyle Width="32px" />
                                                <ItemStyle HorizontalAlign="Center"></ItemStyle>
                                            </asp:TemplateField>
                                            <asp:BoundField DataField="Name" HeaderText="Name" />
                                            <asp:BoundField DataField="mobile" HeaderText="mobile" />
                                        </Columns>
                                    </asp:GridView>
                                    <br />
                                    <br />
                                </center>
                            </ContentTemplate>
                        </asp:UpdatePanel>
                    </div>
                    <div class="modal-footer">
                        <center>
                            <asp:Button ID="btnsend" runat="server" CssClass="btn btn-primary" Text="Send" />
                            <input type="button" class="btn btn-primary" value="Cancel" onclick="window.location = 'SmsAccess.aspx';" />
                        </center>
                    </div>
                </div>
            </div>
        </asp:Panel>
        <asp:LinkButton ID="BTNDummy" runat="server"></asp:LinkButton>
        <ajax:ModalPopupExtender ID="mpesms" runat="server" PopupControlID="pnlsms" TargetControlID="btnDummy"
            BackgroundCssClass="modalBackground">
        </ajax:ModalPopupExtender>
    </div>
    </form>
</body>
</html>
C#
protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        System.Data.DataTable dt = new System.Data.DataTable();
        dt.Columns.AddRange(new System.Data.DataColumn[3] { 
                        new System.Data.DataColumn("Id", typeof(int)),
                        new System.Data.DataColumn("Name", typeof(string)),
                        new System.Data.DataColumn("Mobile",typeof(Int64)) });
        dt.Rows.Add(1, "John Hammond", 1111111111);
        dt.Rows.Add(2, "Mudassar Khan", 2222222222);
        dt.Rows.Add(3, "Suzanne Mathews", 3333333333);
        dt.Rows.Add(4, "Robert Schidner", 4444444444);
        gridsendsms.DataSource = dt;
        gridsendsms.DataBind();
        mpesms.Show();
    }
}
Screenshot
