The article analyzes the principle of the rule of law in international law considering ontological and epistemological aspects of its nature and interrelation with national law. Given the modern understanding of the tasks, purpose and nature of law, both domestic and international, the paper substantiates the view of the rule of law as a moral and volitional basis for the legitimacy of international law. The advantages of this comprehension and perception of the rule of law in the formation of international relations on the basis of anthropocentrism and ethicalcentrism in contrast to the positivist view of international law are considered. The perception of international law throughout the history of its development reveals the immanent characteristics that form an integral understanding of the rule of law in international law as a general principle and as one of its sources, along with international treaties and international customs, general principles, and a judicial precedent. Axiological, anthropological, phenomenological, comparative-historical, dialectical, comparative-legal, systemic-structural, hermeneutic and formal-legal methods were used for the research. The article makes a conclusion that due to the humanistic and valuable principles of the doctrine of natural law, which permeates the principle of the rule of law and is its core, the rule of law becomes capable of serving as the moral and volitional basis of the legitimacy of international law, which gives international law a humanitarian, human-centric and ethical-centric sound and becomes a means of overcoming purely formal, positivist vision of international law, turns it into an important regulator of international relations, an instrument of struggle for human rights and fair relations between states.