Abstract： Starting from the yin- yang theory of traditional Chinese medicine (TCM)， the authors discuss the basic pathogenesis of formation，development and recurrence and metastasis of tumor. Based on the theory of “yang transforms into qi， while yin constitutes form” in Huangdi Neijing (The Yellow Emperor's Inner Classic)，the main pathogenesis of tumor diseases is that insufficiency of yang qi disables the delivery and distribution of yin essence and causes the retention of yin essence in the body， which further leads to blood stasis，phlegm coagulation and qi stagnation. The treatment for tumor should follow the principle of “differentiation of syndrome，time and body” and with attention paid to the reinforce of yang and the occasional dissipation of yin. The proportion of yin and yang should also be identified according to the stage of diseases and the constitution of patients. When adopting the method of warming yang， dissipating masses and dispersing yin，the pathogenesis of yang deficiency should be observed carefully so that the healthy qi will not be damaged. The theory of “yang transforms into qi， while yin constitutes form” can help further enrich and develop the theory of pathogenesis and etiology of tumor in TCM and improve the curative effect of TCM on tumor diseases.