<style>
.round-corner-btn {
-moz-border-radius:4px;
-webkit-border-radius: 4px;
-khtml-border-radius: 4px;
border-radius: 4px;

width:200px; height:40px; line-height: 40px;
background: green;
display:inline-block;
color:white;
text-align : center;
cursor: pointer;
}

.avatar_size{
font-size:12px;
text-align: center;
margin-top:4px;
}

.btn-hover{
opacity: 0.8;
}
</style>
<div class='round-corner-btn'> 按钮</div>
<script>
$('.round-corner-btn').hover(function(){
$(this).toggleClass('btn-hover', 200);
})
</script>