This thesis examined Kim Siseup’s philosophy within the category of the principle and material force theory of Neo-Confucianism, and tried to elucidate the characteristic elements of his understanding of Neo-Confucianism, mainly in light of the cultivating theory, by regarding the theory of ‘human nature is principle’ as a central proposition. Kim Siseup widely accepted the theory of the Great Ultimate, principle and material force, the theory of mind and nature, and the opinion of cultivation focusing on reverence, deliberation of principle, and sincerity, all of which had been established after the North Song Dynasty. In particular, he actively adopted a theory of ‘human nature is principle’ as a core thesis which not only combines the metaphysics of Neo-Confucianism with the theory of mind and nature, but also exposes the essence of Neo-Confucianism as it is. This essay analysed the meaning of the cultivating theory involved in the theory of ‘human nature is principle’ accepted by Kim Siseup through an concrete examination of its contents. Kim Siseup’s opinion regarding the investigation of principle, which serves as its momentum, through the medium of his understanding of the Book of Changes in the process of cultivation from reverence to sincerity was considered. According to this, it can be said that the meaning and characteristics of the cultivating theory involved in the theory of ‘human nature is principle’ to which he refers to embraces the processes of reverence, sincerity and the investigation of principle through the medium of the Book of Changes. In this light, Kim Siseup’s theory of ‘human nature is principle’ can be seen to have mainly embodied, as it is, the fundamental theory of Neo-Confucianism which was built and systematized after the North Song Dynasty. In this point, the writer could evaluate that Kim Siseup considered the core question of the Great Ultimate, the principle and material force theory, the theory of mind and nature, and the cultivating theory that had been discussed within Neo-Confucianism by scholars up to Zhu xi, and that it was Kim Siseup that had consistently synthesized and intensified the Neo-Confucianism of the Joseon Dynasty before Toegae and Yulgok.