International customary law is said to be the father of international law. Accordingly, international crimes are also assumed to evolve out of international customs. Even some international crimes are assumed to have jus cogens status from which no derogation is allowed. At the other angle, however, there is the principle of legality in the criminal law regime which requires strict time reference from which retroactive application of criminal laws is prohibited, precise provisions clearly putting prohibited acts, written provisions to refer such prohibited acts etc. Yet international customary law does not have the capacity to create international crimes fulfilling all these features of the principle of legality. On the other, the principle of legality is the principle which is accepted almost by all states of the world. Therefore, it is impossible for the international customary law to create international crimes duly accepted.