on the one hand,on the other hand后面都要有“,

首先你的问题中给出的短语是错的,是On one hand/On the other hand,第一个里面不能加the
这一组短语有两种不同的用法,当把它们放在句首时是必须加逗号的.但是另外一种用法是放在句尾,此时不用加逗号（都句尾了也没法加逗号了）.我举个简单例子吧：
On one hand,building a factory would do some good to us.But on the other hand,its side effects are still doubted to be unharmful.
Building a factory would do some good to us on one hand.But its side effects are still doubted to be unharmful on the other hand.
以上两种表述都是对的,这是托福考试作文里的常用句,你可以自行体会
希望对你有所帮助