In order to investigate the effect of carbon ion irradiation on apoptosis and Bax/Bcl-2 expression in human tongue carcinoma cells, exponentially growing human tongue carcinoma cells (Tb) cultured in vitro were irradiated with 0, 0.5, 1.0, 2.0 or 4.0 Gy of ,12,C,6+, ions respectively. Survival rate of irradiated cells at various doses were measured by MTT assay. The nucleus changes of apoptosis and necrosis of cells stained by Hochest/PI were observed through fluorescence microscope. The cell cycle changes were detected by flow cytometry (FCM). The expressions of Bax and Bcl-2 were detected by Western blot analysis. The results show that the viability of Tb cells decreases gradually with increment of irradiation doses of carbon ions. The proportions of apoptosis cells in the irradiated groups are significantly higher than those in the control group. There is a positive correlation between irradiation doses and retardation strength in G,2, /M phase at 24 h after irradiation (,P,<,0.05). And the expressions of Bax and bcl-2 are significantly up-regulated and down-regulated respectively by ,12,C,6+, ion irradiation. It can be concluded from above that cell apoptosis induced by heavy ion with high-LET may be mediated through the Bax/Bcl-2 expression pathway.