script type="text/javascript">
$(document).ready(function () {
$("#<%=rdoCountry.ClientID
%>").change(function () {
var countryid = $('#<%=rdoCountry.ClientID
%>
input[type=radio]:checked').val();
alert(countryid);
});
});
</script>
<asp:RadioButtonList ID="rdoCountry" runat="server">
<asp:ListItem Value="1">India</asp:ListItem>
<asp:ListItem Value="2">USA</asp:ListItem>
<asp:ListItem Value="3">UK</asp:ListItem>
<asp:ListItem Value="4">Japan</asp:ListItem>
</asp:RadioButtonList>