The question of "Why Be Moral" is an important issue in meta-ethics. In this article I will start with the examination of the empirical approach to this question and its difficulties. I will then show that how the discussion is inevitably led to the sphere of metaphysics. Mou Tsung-san argued that only the Moral Metaphysics of Confucianism can provide a substantial ground for morality. He explained the universality and necessity of morals by the Confucian idea of original heart-mind and original nature. Apart from elaborating Mou's view, I also expound that in what sense there is a close relation between moral philosophy and metaphysics.