Required field validator for CheckBoxList

<asp:CheckBoxList ID="chkDepartments" runat="server" RepeatDirection="Horizontal"></asp:CheckBoxList>

                <asp:CustomValidator runat="server" ID="chkT"

                    ClientValidationFunction="ValidateDepartments"

                    ErrorMessage="Please select atleast one Department" ValidationGroup="k" ForeColor="Red"></asp:CustomValidator>

<script type="text/javascript">

        function ValidateDepartments(source, args) {

            var chkListModules = document.getElementById('<%= chkDepartments.ClientID %>');

            var chkListinputs = chkListModules.getElementsByTagName("input");

            for (var i = 0; i < chkListinputs.length; i++) {

                if (chkListinputs[i].checked) {

                    args.IsValid = true;

                    return;

                }

            }

            args.IsValid = false;

        }

    </script>

Hope this will helps you................