Terrorism and terrorist organizations are important players in international relations. Terrorist attacks and the reaction of states against these attacks have contributed to the discussion of the subject of recourse to force against acts of terrorism. The present article aims to answer the following questions regarding this issue: What is the view of customary international law with regard to the legitimacy of recourse to force in combating terrorism? Following the extensive interpretation of the article 51 of the charter of the United Nations which legitimizes the use of force in combating terrorism, what would be the confrontation between two extensive and intensive interpretations of the mentioned article? To answer these questions, the view of customary international law on the subject of legitimacy of use of force in combating terrorism will be analyzed.