The article presents Aristotle's and H. L. A. Hart's approaches to indeterminacy of law. It is shown that both Hart and Aristotle have associated indeterminacy with the general nature of law and legal rules, but they interpret this relationship in different ways, as well as in different ways they interpret the reasons of general nature of law and indeterminacy. If for Aristotle the general nature of law is the cause of indeterminacy, then for Hart it is a consequence. But both philosophers definitely agree that for a more effective legal regulation, a margin of indeterminacy in legal rules should nevertheless be tolerated.