Scholars have different views on Zhu Xi’s moral philosophy based on their own interpretive approaches. This chapter suggests one particular approach to understand and reinterpret Zhu Xi’s core concepts related to moral practice. We focus, in particular, on clarifying the relationship between mind (xin 心) and moral principle(s) (li 理) in accordance with his original texts. On the basis of our interpretations of the original texts, we will address some controversial issues derived from various modern interpretative approaches to Zhu Xi’s studies. This chapter intends to focus our research on interpretations of Zhu Xi’s moral claims made by contemporary Confucian philosophers, Mou Zhongsan and Tang Junyi. One important reason is that their interpretations are constructed on the basis of precise philosophical analyses and the study of a great deal of texts. Another factor is because of their great influence on modern scholars of Sinophone and Anglophone academia in the twentieth and twenty-first centuries. Mou and Tang shared some perspectives on Zhu’s philosophy. However, their understandings of the core idea in Zhu’s moral theory, “how mind and moral principle(s) are related,” are quite different. The issues concerning “how mind and moral principle(s) are related” will be addressed in this chapter. We introduce and discuss the main interpretations of Mou and Tang regarding Zhu’s moral philosophy. Then, we will also provide a brief sketch of Cheng Yi’s 程頤 moral theory because of his great influence on the development of Zhu Xi’s philosophy. Based on these discussions, we offer our conclusions that (1) Zhu Xi’s moral philosophy is the “Model of Priority in Reflecting on Moral Principle(s)” (zhulixingtai 主理型態); (2) The primary purpose of investigating moral principle(s) of things (gewu zhizhi 格物致知) is to accomplish a philosophically rational understanding of morality, which means that one has to elevate the “ordinary rational understanding of morality” (changzhi 常知) to a level of a “philosophically rational understanding of morality” (zhenzhi 真知); (3) Tang’s interpretations better conform to Zhu’s original claims.