​x
 
<html>
<head>
<script type="text/javascript">
function clickCheckbox()
  {
  document.getElementById('check1').click()
  }
</script>
</head>
<body>
​
<form>
<input type="checkbox" id="check1" />
<br />
<input type="button" onclick="clickCheckbox()" value="Click checkbox" />
</form>
​
</body>
</html>
​