i have 2 radio button. how can keep one button by default checked

You just need to provide checked="checked"

for e.g:

<form id="form1" runat="server">
    <input id="rad1" type="radio" checked="checked" />
    <input id="rad2" type="radio" />
</form>

Here the first Radio button will be checked at run time whereas the second one will be unchecked