<form action="index.php" method="post">
<select name="sName" id="s_id">
<option value="aaa">AaA</option>
<option value="bbb">BbB</option>
<option value="ccc">CcC</option>
</select>
</form><script type="text/javascript">
document.getElementById('s_id').value = 'bbb';
</script>;