look at表示“看，考虑”的意思；look up表示“查字典，拜访，仰望”的意思。虽然都是look 和介词组成的词组，但是两者在含义上有很大的区别。
look at 动词词组， 表示“看；考虑；着眼于”。
1）看；朝…看；盯
The children looked at their new teacher in curiosity.
孩子们好奇地望着新老师。
2）看待；认为：
We should learn to look at problems all-sidedly.
我们应该学会全面看问题。
3）考虑;着眼于;接受[常用于 will(或 would) not look at]:
The manager will not look at a report like that.
经理不会答理那样一份报告。
4）检查；察看：
You should look at that house before you decide to buy it.
你应该看看那栋房子再决定购买。
5） 过目；浏览：
Have you looked at their report?
你看过他们的报告了吗?
2、look up 动词词组，表示“仰望；查阅；尊敬；拜访”的意思。
1）查询，查看
It is unnecessary to look up every word you don’t know in the dictionary while your are reading.
阅读时,碰到生词,不必都要在词典中查阅。
2）抬头看，好转
I looked up and saw him.
我抬起来看见了他.
The national economy is beginning to look up.
国民经济正在开始好转.