<!doctype html>
<html>
<head>
<meta charset="utf-8" />
<title></title>
<script src="https://code.jquery.com/jquery-1.11.1.min.js"></script>
</head>
<body>
<iframe id="fr1"></iframe>
<button>Change URL iframe</button>
<script>
var urls = ["http://javascript.ru", "http://javascript.ru/forum", "http://javascript.ru"], i = 0;
$("iframe").load(function () {
alert("load");
});
$('button').click(function() {
$('#fr1').attr('src', urls[i % urls.length]);
i++;
});
</script>
</body>
</html>