<input type="radio" name="sex" value="Male">Male</input>
<input type="radio" name="sex" value="Female">Female</input>
<input type="radio" name="sex" value="Unknown">Unknown</input>

$('input:radio[name=sex]:checked').val();

$('input:radio[name=sex]:nth(0)').attr('checked',true);
或者
$('input:radio[name=sex]')[0].checked = true;

$('input:radio[name=sex]:nth(1)').attr('checked',true);
或者
$('input:radio[name=sex]')[1].checked = true;

$('input:radio[name=sex]:nth(2)').attr('checked',true);
或者
$('input:radio[name=sex]')[2].checked = true;

$('input:radio[name=sex]').attr('checked',false);