<style type="text/css">
html, body { margin: 0; padding: 0; height: 100%; }
iframe {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
border: none;
box-sizing: border-box;
-moz-box-sizing: border-box;
-webkit-box-sizing: border-box;
}
</style>
<iframe src="http://www.google.com/"></iframe>
參考：http://stackoverflow.com/questions/1303729/iframe-100-height-inside-body-with-padding