<script>
function checkAll(c){
var chks = document.getElementsByName(c.name);
for(var i=0;i<chks.length;i++){
chks[i].checked = c.checked;
}
}
</script>
<input type="checkbox" name="myCheck" onclick="checkAll(this)">
<input type="checkbox" name="myCheck" onclick="checkAll(this)">