.main{
height:500px;
background: #555;
position: relative;
}
.item{
width:100px;
height:100px;
background: #ff5;
text-align: center;
position: absolute;
top:50%;
left:50%;
margin-left: -50px;
margin-top: -50px;
}<div class="main"> <div class="item"></div> </div>