In this article the author first discusses the Ch'eng brothers' moral theory, Which is closely connected with their ontology. Ch'eng Hao maintains that heaven and man are from a single root, but distinguishes between the truthful and the false at the side of man. Moral practice, by which man behaves truthfully, is therefore indispensable. Another point of his ontology is the continuation of heavenly virtue; moral practice consists then in the gradual and endless correction of oneself so as to become closer to the purity of heavenly virtue. In his ethics there is also a combination of heavenly virtue with good government. Ch'eng I's ontology is different from his brother's in that substance is seen as a perpetually-unmoved One. Moral practice is for him to discern and extinguish selfish desires; it is by such continual self-conquering that one returns to the objective and uninterested principle. The author then discusses their theories of cultivation, including “holding to attentiveness” and “investigating principle”. The cultivation of “holding to attentiveness” for Ch'eng Hao results in linking oneself to the continuation of heavenly virtue. As for Ch'eng I, this effort changes man from within. He argues that “holding to attentiveness” should start from orderliness and seriousness of behaviour, and then advance to the stage where the mind masters itself. The significance of “investigation of principle” is also different for the two brothers. For Ch'eng Hao it is a practice subsidiary to “holding to attentiveness,” and is for the purpose of establishing the correct interaction between man and his environment so as to ensure the continuation of heavenly principle. Ch'eng I, on the other hand, regards “investigation of principle”as an effort to access the eternally－unmoved principle; therefore it is a step further and more important than “holding to attentiveness”.