学习了，不错

window.onload=function(){

var oDiv=document.getElementById('div1');

oDiv.onmouseover=function(){

startMove(100);

};

oDiv.onmouseout=function(){

startMove(30);

}

};

var timer=null;

var alpha=30;

function startMove(iTarget){

var oDiv=document.getElementById("div1");

clearInterval(timer);

timer=setInterval(function(){

var speed=0;

if(alpha>iTarget){

speed=-10;

}

else{

speed=10;

}

if(alpha==iTarget){

clearInterval(timer);

}

else{

alpha+=speed;

oDiv.style.filter='alpha(opacity:'+alpha+')';

oDiv.style.opacity=alpha/100;

}

},30)

}