The crime of aggression is the most paradoxical criminal offence in the International Criminal Law. On the one hand, it is deemed as the severest crime in the international society, the "legitimacy" of its severe punishment is rarely questioned. On the other hand, there is still no consensus for a precise definition on the crime of aggression, which means, the "legality" of its punishment is still an issue needed attention. Though "The crime of aggression" is clearly stated in the sub-paragraph d of paragraph 1 of Article 5 in the Rome Statute of the International Criminal Court passed in 1998, the content of this crime remains empty; therefore the International Criminal Court has no jurisdiction over the crime of aggression. But at least we can say, the aggression as a crime in the International Criminal Law is agreed in the United Nation's Assembly. And many documents drafted by the International La Commission of the U.N have admitted this crime, which also finally included in the Rome Statute of the International Criminal Court. Therefore, the crime of aggression is undoubtedly an admitted crime in the Customary International Law.