Edit in JSFiddle

div
{
    width: 200px;
    height: 200px;
    background: #33c;

    -webkit-border-radius: 50%;
    -moz-border-radius: 50%;
    border-radius: 50%;
}
.second
{
    -webkit-border-radius: 100px;
    -moz-border-radius: 100px;
    border-radius: 100px;
    background: #c33;
}

<div></div>

<div class=second></div>