function ValidateCheckBox(sender, args) {
var checkbox = document.getElementById("%=CheckBox1.ClientID %>")
if (checkbox.checked) {
args.IsValid = true;
}
else {
args.IsValid = false;
}
}

asp:CheckBox ID="CheckBox1" runat="server" />
asp:CustomValidator ID="CustomValidator1" runat="server" ErrorMessage="必須選擇選項" ForeColor="Red" ClientValidationFunction="ValidateCheckBox">/asp:CustomValidator>br />
asp:Button ID="Button1" runat="server" Text="提交" />