Abstract: BACKGROUND ＆ AIM： To study the oxidative effect of H2O2 on HepG2 cells in different phases of the cell cycle to explore the apoptosis mechanism of cancer cells under oxidative stress. MATERIALS AND METHODS：Synchronized cells were collected by arresting with TdR, then cells at different phases of cell cycle were treated by 800 μmol/L H2O2 and content of MDA, activities of XOD, 5'NT , anti- O2 and anti-•OH were measured. RESULTS： The content of MDA and activities of XOD, anti-•OH,anti- O2 increased and activities of 5'NT decreased significatly in cells treated with H2O2 . Compared with non-synchronized cells, the content of MDA and activities of XOD, 5'NT，anti-•OH and anti- O2 increased markedly in synchronized S-phase cells and G2/M-phase cells .The content of MDA and activities of XOD, 5'NT， anti- O2 were significantly lower in synchronized G1-phase cells. CONCLUSION： When synchronized cells were treated with H2O2, the degree of lipid peroxidation, activities of enzyme that catalyse production of free radicals， and level of anti-oxidation were all associated with the cell cycle. This may be due to different sensitivities to oxidative damage induced by H2O2 at the different phases of the cell cycle.

Key words: cell cycle, H2O2, oxidative effect, HepG2 cell