Rationalism is a seventieth century philosophical thought that evaluates issues on the basis of reason as the only valid resource of knowledge and has empiricism as its opponent view. Generally, the school of thought emphasizes on the primacy of reason in different sciences inter alia the international law. Given that reason has provided a basis for rapid and widespread developments in international law, the main issue of this article is to study and evaluate the impact of rationalism on the current bases and order of international law. Accordingly, the impacts of rationalism on international law and its emergence as the battle field of tradition and modernization will be considered in this context. It is obvious that rational approaches have influence on different branches of international law and many important international rules and documents are created according to it. The main aim of this article, which is based on a descriptive-analytical method and library studies, is explaining the rational effects on states’ approaches and practices in international law, and then justify some current developments in international law on the basis of reason. The results of the studies indicate that in different branches of international law, while the transmission and flow of the rationalist approaches is confirmed, in practice, also the legal rationalist traces can be clearly seen in the practice of the states, international organizations and international dispute settlement bodies, although there is a serious confrontation between reason and the reality of the world community.