我假设你已经使用了height属性，所以我把它与一个高度比较
left with javasript

I am assuming that you have used height attribute at both so i am comparing it with a height left do it with javasript

var right=document.getElementById('rightdiv').style.height;
var left=document.getElementById('leftdiv').style.height;
if(left>right)
{
    document.getElementById('rightdiv').style.height=left;
}
else
{
    document.getElementById('leftdiv').style.height=right;
}

另一种想法可以在这里找到
HTML / CSS：使两个浮动div具有相同的高度

Another idea can be found here HTML/CSS: Making two floating divs the same height

请注意，此代码未经测试

note that this code is not tested

感谢我希望这将帮助您......关注... ：）

Thanks i hope that this would help you ......regards...:)