Immanuel Kant showed that morality is based on a single supreme universal principle which is binding to all rational beings. Precisely, Kant wanted to establish the first principle of morality which neglects all consideration of self-interest and particular human problems. He put this position in his work, Groundwork of the Metaphysics of Morals. So, in this book, I mainly focus on the doctrines of Groundwork of the Metaphysics of Morals. In Groundwork of the Metaphysics of Morals, Kant claimed that his intention is to seek out and establish the supreme principle of morality, and that supreme principle is the categorical imperative. This is, for him, the only supreme principle for all fields of morals. He attempted to justify this at the end of Groundwork of the Metaphysics of Morals. But, the way he attempted to justify the categorical imperative is problematic. In this book, I will make clear that Kant did not put categorical imperative or morality on a solid ground. Thus, thisbook is especially useful for anyone who is interested to study Kant's ethics and wants to understand the limitation of Kant's idea of morality in Groundwork of the Metaphysics of Morals.