Our previous study showed that transmembrane TNF-α (TM-TNF-α) had broader tumoricidal spectrum than secretory TNF-α (s-TNF-α). This study examined the difference between the two kinds of TNF-α in inducing cells and the relationship between the apoptosis induced by TM-TNF-α and the cell cycle. Bioassay was employed to compare the cytotoxic effect of two kinds of TNF-α on cell lines L-929 and HepG2. TUNEL was used to detect apoptosis and the TdT and PI co-stainin gwere used for determining the phase of apoptotic cells. Our results showed that TM-TNF-α could kill not only s-TNF-sensitive L929 cells but also s-TNF-tolerant HepG2 cells. TM-TNF-α predominantly induced apoptosis while s-TNF could induce both apoptosis and necrosis. The apoptosis of L-929 cells induced by TM-TNF-α mainly occurred in S phase and the apoptosis of HepG2 predominantly took place in G1 phase. It is concluded that the cytotoxic effects of the two TNF differ substantially.Since TM-TNF-α works locally, mainly induces apoptosis and has broader anti-tumor spectrum, it may be more effective for the treatment of tumor than s-TNF.