Zhu Xi was a synthetic thinker and a preeminent scholar of Neo-Confucianism in Song Dynasty. Following early tradition and based on Chen Yi and Cheng Hao's theory, he associated humanity with cosmic creativity and regarded the metaphysical synthesis as his own duty. He became the leading figure of the School of Principle and the most influential rationalist. From the perspective of Zhu Xi's ontology, his treatise had a two-pronged approach including human nature and Principle (Li). The Li theory was the basis of the theory of human nature, while the theory of human nature was the conclusion of Li Principle. This article elaborates the mind of human and the virtue of humanity of Zhu Xi's philosophy of human nature, the personality, the moral nature, as well as the nature of heaven and earth for exploring the mystery of the Zhu Xi philosophy.