道德原則的實踐性是康德道德哲學中一個重要的問題。如果道德原則不具有實踐性，康德義務論就很難成立，甚至危及到道德的實在性。事實上，這一實踐性不僅存在且有著豐富的內涵。第一，道德原則根源於作為行為根據的自由性。道德原則是自由的法則，是意志自我立法而強加給我們自身的一種道德必然性。遵循道德原則的行動是「通過自由而可能」的活動。第二，道德原則是實踐知識。康德的道德原則是作為實踐命題提出來的，它是關於人們應該如何行動的知識。第三，道德原則是一種實踐模式。其程序是純粹實踐理性規定人的意志，使他（她）採用道德法則作為準則去行動。第四，道德原則在德性義務、行為動機和道德判斷方面產生了主客觀的效應。

The practicality of the moral principle plays an important role in Kant’s moral philosophy. It is critical for Kant’s deontology to set up, and even endangers the moral objective reality. In fact, this practicality not only exists, but also has the rich connotation. First, the moral principle is rooted in freedom. Kant’s moral principle is the law of freedom, it is a moral necessity which is our will imposes on itself by self-legislation. Following the principle is a possible activity by freedom. Second, the moral principle is practical knowledge. The moral law is the innate practical knowledge, which requires that we only act according to this norm that can become a universal law. Third, the moral principle is a kind of mode of practice. The pure practical reason dominates the will to make him (her) act or perform duty in the norm as the moral law. Fourth, the moral principle has a subjective and objective effect on moral duty, motivation, and the moral judgment.