Customary international law includes those rules that are established and sustained by the consistent and uniform practice of states and other subjects of international law in their international legal relations, in circumstances that give rise to legitimate expectations of the application of a similar practice in the future, when a large number of states participate in such practice on a permanent and continuous basis as specific behavioral rules. Since, according to Article 77 of the Constitution of the Islamic Republic of Iran; international treaties must be approved by the Islamic Consultative Assembly and, according to Article 94, all resolutions of the Islamic Consultative Assembly must be sent to the Guardian Council for evaluation in accordance with Islamic standards and the Constitution. Thus, in reviewing international treaties, the Guardian Council sometimes encounters customary international law stipulated in the preamble and text of the treaty. This article seeks to answer the question of what is the Guardian Council's practice in dealing with customary international law and how can it have an effective confrontation? The present article, using a descriptive-analytical method, reaches the conclusion that, first, the Guardian Council, when confronted with the rules of customary international law in general, considers them to be contrary to Islamic law and the constitution; second, by accepting the generalities of customary international law and the existing proposed solutions, it can have an effective confrontation in this regard.