Abstract: Institztte of medical Biochemistry，Gzcangdong Medical College, Zhanjiang,Genistein is a isoflavone with antioxidant property and antitumor activity.hydroxyguanine is a sensitive marker for the oxidative damage to DNA，and a well-estabilished biomarker relevant to carcinogenesis and aging as well .To study the relationship between the antioxidant property of genistein and its antitumor activi-ty，in this paper，the effect of genistein on the damage to guanine induced by Fen-ton reaction 2O₂+Fe²⁺·OH+OH-+Fe³⁺)was studied using capillary gas chromatography/mass spectrometry. The results show that genistein can inhibit the formation of hydroxyguanine potently，and at a concentration of 60y.mol·Lgenistein can reduce the production of 8-hydroxyguanine to of that in control，which suggest that the inhibitive effect of genistein onH₂O₂ induced DNA damage maybe implicated in its antitumor ctivity.Based on this paper and other studies，it is suggested that this inhibitive effect of genistein results from the scavenging ofH₂O₂ by genistein.