The Comparative law is useful for the comprehension of foreign peoples and it gives a better regime to international relations. This interest does firstly affect the International Public Law. In fact, the conditions of the modern world do impose a total renewal of this branch of law. It must settle among states, the relations of cooperation at all levels. It is clear that these relations may not settle or develop as required due to the ignorance of international law. The UNESCO constitutive act recognizes the knowledge and the mutual comprehension of nations by the development at the universal level of the study of the foreigners law through comparative method. For this reason, diplomats and treaties negotiators must be prepared to understand the legal systems  of the concerned parties in order to know in which way and for which arguments they may hope to convince their interlocutors.