var formobj = document.forms[0];

var counter = 0;
for (var j = 0; j < formobj.elements.length; j++)
{
    if (formobj.elements[j].type == "checkBox")
    {
        if (formobj.elements[j].checked)
        {
            counter++;
        }
    }       
}

alert('Total Checked = ' + counter);

.

使用JQuery：

alert($('form input[type=checkBox]:checked').size());