 <p>You need to add "px" to left. </p> <pre><code>bouger.style.left= left + "px"; </code></pre> <p>Of course, it also runs so fast you can't see anything happen, so I have a modified version:</p> <p><a href="http://jsfiddle.net/5GEru/2/" rel="nofollow">http://jsfiddle.net/5GEru/2/</a></p> <p>Also, your code should terminate at some point. Like when left becomes greater than SOMETHING for example. If you're using jQuery of course, you could just do:</p> <pre><code>$(function(){ $("#bouge").animate({"left": 500},2000); }); </code></pre> <p>See: <a href="http://jsfiddle.net/5GEru/6/" rel="nofollow">http://jsfiddle.net/5GEru/6/</a><br> ​</p>