The understanding of tumor etiology in Chinese medicine is limited to six climatic external pathogens

improper diet

overstrain

and internal damage emotions

etc. Pathogenesis interpretation is mainly Qi stagnation

blood stasis

obstruction by phlegm

dampness-retention

toxin

weakened body resistance

etc. This understanding basically follows the pathogenesis theory "accumulation diseases"in traditional Chinese medicine and cannot explain the essential characteristics of tumor unlimited growth. This paper discusses the process of tumor occurrence and development by Yin-Yang differentiation combining local focus with whole body. The authors hold that disorder of interaction of Yin-Yang is the reason of low grade tendency of tumor focus structure and function. Yin-Yang disorder of Qi movement is the key of pathological product formation. This paper explores the possibility of tumor reverse and provides new thinking for treating tumor with Yin-Yang theory.