This paper is a tentative evaluation of yang Kueishan’s philosophy of principle (li-hsueh) and its historical position in the development of Sung Neo-Confucianism as a whole. It seems (1) to demonstrate the Yang Kuei-shan’s thought served as an important bridge between the two Cheng Brothers and Chu Hsi, and (2) to explain the actual content of Yang’s philosophy as a process of sythesis drawing from Confucianism, Taoism and Buddhism. The approach of the argument, therefore, can be divided in the following four categories:1.Historical: the original source of his philosophy.2.Ontological: the principle of ying and yang, the transformation of material force (ch’i), and the Way of Heaven (t’ien) and its mandate.3.Epistemological: the investigation of things in the extension of knowledge, and the principle of one-in-all.4.Ethical: the concept of self-cultivation and the contemplation of equilibrium (chung).In the analysis of Yang’s philosophy, this paper also attempts to deal with his sivotal methodology of “directness”(chih) in an effort to explicate his doctrines of loyalty and altruism, the one source of internality and externality, the manifestation of nature through the human mind, and the grasping of principle in the midst of human affairs.