[AndrewGSW]

I can't see your div with id of 'content', but presumably it is there somewhere.

From jQuery 1.6 prop() is intended to specifically return a property, so you should use attr() to return an attribute.

$(function () {
    $('#ul1 li a').on('click', function(e){
        e.preventDefault();
        var page_url=$(this).attr('href');
        $('#content').load(page_url);
        return false;
    });
});

on() was added in jQuery 1.7 so check the version that you are using.

preventDefault() should work, to prevent the default link being followed, but there is no harm in including 'return false' as well.