It is usually said that the main point of Kant's thought is contained in 'Critique of Pure Reason". But it may be, I think, advocated that, examining Kant's essential intention flowing through his philosophy, 'Critique of Pure Reason' is a preperatory book for 'Critique of Pure Reason'. He emphasized the superiority of practical reason to theoretical reason. Accordingly many philosophers get interested in his practical philosophy more than in his theoretical one in recent times. Generally Kant's ethical thought has heen known as formal one, bnt now it is maintained by some prudent philosophers that his ethical thought is not merely formal but real (material) namely that Kant's intention, which had cleared formal principles in detail, aimed at solving real problems in human life according them. Thus, investigating his works more carefully, we can see that while ' Fundamental principles of the Metaphysics of Morals' and 'Critique of Pure Reason' of his three great works of practical philosophy are the works having cleared formal principles, Metaphysics of Morals', the rest one of the three, is the work of real(material) moral having applied them to real life. In this thesis (1) three fundamental spirits of Kant's ethical thought were extracted (2) the transition from the theoretical philosophy to the practical philosophy was traced (3) the fundamental formal principle of moral, the categorical imperative was considered (4) the real(material) theory of moral applying such formal Principles to human real life was examined.