It can be done using the border-radius
property. basically, you need to set the border-radius to exactly half of the height and width to get a circle.
HTML
<div id="container">
<div id="inner">
</div>
</div>
CSS
#container
{
height:400px;
width:400px;
border:1px black solid;
}
#inner
{
height:200px;
width:200px;
background:black;
-moz-border-radius: 100px;
-webkit-border-radius: 100px;
border-radius: 100px;
margin-left:25%;
margin-top:25%;
}