One of the most important epistemological challenges of international law to answer the question: Is international law really a Science? It is an issue that has never been addressed in jurisprudence and has been neglected, in view of the fact that law philosophers efforts have basically been made to prove legal aspects of International Law. Current vacancy has made the writer analyze the theoretical and scientific structures of International Law in light of philosophy of science developments. By describing demarcation standards of science from non science has been referred to logic of causal based. Meaning based and reason based explanations; in so that empirical international law uses the same causal explanation patterns that a science such as physics and analytic methodology is usable in the same proportion that International Law is loyal to experiment. Nevertheless, similarity of explanation patterns and methodology styles shouldn’t be ignored from the epistemological differences of International Law. In addition to causal explanation logic, reason based explanation pattern is needed to justify and obtain optimum understanding of legal phenomena. These different explanation patterns proposes a methodological variety that we don’t suffice verifiable and falsifiable methodologies in the format of unity of methodology, nevertheless methodological pluralistic is a must that progressive and developing science of international law is intensive need of it.