Abstract: The premise and basic view of Kant’s moral philosophy is that the law of morality should possess both absolute necessity and universal validity.Due to the necessity and universality,the ethic factor of happiness theory is excluded and the fact is proved that the concept of freedom is rational.However,it has long been argued whether this law does exist and whether it is practically significant.Research shows that since Kant has premised the realm of freedom of the will and purpose,he would have premised the universal necessity of the law of morality;and the consciousness of the law of morality of universal necessity in turn has proved the existence of freedom.Kant’s law of morality,as the moral evaluation and criterion for the standard of behavior,emphasizes the priority of rationality and keeps experience open as well,leaving enough space for the creation of moral practice and the development of philosophy itself.

Key words: Kantism, absolute necessity, universal validity, law of morality