When God has created animals, he saw "it was good" (Genesis 1:25). Then why he told Noah that there are unclean animals (Genesis 7:2)? Aren't all animals created by God and "it was good"? What makes some animals clean while others not?
If you look at the list of clean versus unclean animals given in the Law of Moses you will see a pattern. Unclean creatures are those which eat dead and decaying things as a part of their diet. See Uncleanness for more details.
Thank you for your answer. At the time of the flood, there was no Law of Moses, then how do we know that the list unclean animals are the same for Noah and the Law of Moses?
The first five books of the Old Testament are attributed to Moses. Though the Bible never says Moses wrote Genesis, it is generally understood that he served at least as the editor. Given that, though the words clean and unclean were precisely defined in the law of Moses, the understanding of the terms would contain the same meaning as understood by Moses and the Israelites. For example, you will find people arguing that perhaps "day" meant something different during the creation, but since Genesis was edited by Moses, the term would have to have the same meaning as the Israelites understood it during the time of Moses.