You can use CSS3 media query for this. Write like this:
CSS
.wrapper {
border : 2px solid #000;
overflow:hidden;
}
.wrapper div {
min-height: 200px;
padding: 10px;
}
#one {
background-color: gray;
float:left;
margin-right:20px;
width:140px;
border-right:2px solid #000;
}
#two {
background-color: white;
overflow:hidden;
margin:10px;
border:2px dashed #ccc;
min-height:170px;
}
@media screen and (max-width: 400px) {
#one {
float: none;
margin-right:0;
width:auto;
border:0;
border-bottom:2px solid #000;
}
}
HTML
<div class="wrapper">
<div id="one">one</div>
<div id="two">two</div>
</div>
Check this for more http://jsfiddle.net/cUCvY/1/