The notion of practical syllogism occupies a crucial place in contemporary debates on human action. The practical syllogism is a form of practical reasoning expressed in syllogistic form, in which a human action is defined as a conclusion drawn from major and minor premises. This notion was not invented by modern or contemporary philosophers; it was Aristotle who first applied the concept of practical syllogism to account for the nature of our actions. What Aristotle meant by this notion, however, is unclear. 　The present article aims to clarify the meaning of the notion of practical syllogism in Aristotle’s works, such as De Anima, De Motu Animalium, and Nicomachean Ethics. In these works, he divided syllogism into two patterns: the means-end pattern and the rule-instance pattern. Most previous studies focused on the former pattern and did not address the significance of the latter. In this article, I first explain Aristotle’s idea of practical syllogism in general. Then, focusing on the rule-instance pattern, I will show that this pattern of practical syllogism plays an important role in Aristotle’s theory of human action, particularly in his Nicomachean Ethics. According to Aristotle, children and young people can become good persons only after they undergo moral development by respecting the rule-instance pattern practical syllogism. In setting forth my argument, I also address the divergence between Aristotle’s notion of rule and the Kantian idea of duty as, in his development of this notion, Aristotle did not consider the Kantian idea of duty that is independent of particular actions. For him, the rule must be inseparable from particular human actions, amounting to that which people, who want to become good persons, should learn and establish to carry out their own actions.