The authors consider the meaning of the concept of rationality in law. They discuss the concepts of formal and substantive rationality developed by Max Weber in their application to the field of legal reality. The most crucial point here is while the formal rationality provides structural consistency of legal norms and unity of legal interpretations, the substantive rationality is focused on the social purposes of law enforcement. The authors consider “the pure theory of law” by Hans Kelsen in its relation to the idea of justification of the formal rationality in legal theory. They pay special attention as well to some prospects and risks of the “materialization” of rationality in modern law. They claim that we could avoid some risks rising from “the materialization” of law if we put some local goals and purposes of law enforcement into the context of global principles of social organization. The authors argue that the integration of the basic principles of Marx’s and Weber’s social analysis could provide the research with some useful insights on the origins of “substantive” rationality in modern law. The authors try to show that, despite all the theoretical contradictions between Marxian and Weberian approaches, these concepts seem to be compatible in the field of legal rationality.