The article analyzes equally important both for philosophy and law issues of rationality, considers the categories of reasonableness of private law, their role and significance in the legal regulation. The relevance of the study is determined by the lack of clear criteria for determining the nature and character of these categories, by the diversity of understanding of their content, role and significance in the process of legal regulation. The purpose of the study is to establish the nature, interconnection and interdependence of rationality, reasonableness of private law, show how they differ from close-by categories of justice, fairness, legality and appropriateness. In this regard, the study used system-logical, formal-dogmatic, functional, comparative-legal research methods. The study draws attention to the essential differences between rationality and reasonableness, which today are often identified, specifies clear relationship of reasonableness, justice and fairness, and at the same time emphasizes their autonomy, synonymy of rationality, meanings underlines the criteria of rationality and reasonableness, emphasizes the irreducibility of reasonableness to the legitimacy, highlights the signs inherent to reasonableness, which are appropriateness and legitimacy. Reasonableness is not only a subjective category, but also, above all, is a requirement objectively expressed in positive law, addressed to all the participants regulated by the law of public relations, i.e., an objective category. In its turn, certainty of the law is expressed as its reasonableness. The study traces the manifestation of reasonableness category in the international legal acts and normative legal acts of a number of national legal orders.