<script type="text/javascript">
        var TargetBaseControl = null;
        window.onload = function()
        {
            try
            {
            TargetBaseControl = document.getElementById('<%= this.grid_reneval.ClientID %>');
            }
            catch(err)
            {
                TargetBaseControl = null;
            }
        }
        
        function TestCheckBox()
        {              
            if(TargetBaseControl == null) return false;
            var TargetChildControl = "CheckBox1";
            var Inputs = TargetBaseControl.getElementsByTagName("input"); 
            for(var n = 0; n < Inputs.length; ++n)
            if(Inputs[n].type == 'checkbox' && Inputs[n].id.indexOf(TargetChildControl,0) >= 0 && Inputs[n].checked)
            return true;			     

            alert('Select at least one !');
            return false;
        }


    </script>


<asp:TemplateField HeaderText="Member">
                                                                                        <ItemTemplate>
                                                                                            <asp:CheckBox ID="CheckBox1" runat="server" Text=" " />
                                                                                        </ItemTemplate>
                                                                                        <HeaderStyle HorizontalAlign="Left" />
                                                                                        <ItemStyle HorizontalAlign="left" />
                                                                                    </asp:TemplateField>