In the present study, the effects of onion and garlic on copper and 1, 10-phenanthroline com plex induced DNA degradation were investigated by the decreased level of thiobarbiturin acid (TBA) reactive materials. Phenanthroline is specific for copper and the reaction releases TBA reactive material from DNA which can be measured by absorbance at 535nm. The levels of TBA reactive materials were decreased by adding onion or garlic ghomogenate into reaction mixture but the onion had more strong potency and the effect of onion was not changed by boiliing. Superoxide dismutase (SOD) and catalase have no inhibitory effects on copper induced DNa damage but reduced glutathone was more effective. The activities of antioxidant enzymes and the contents of sulfhydryl groups in onion and garlic were also investigated. The activity of SOD was more higher in garlic, but catalase and glutathione peroxidase activities were higher in onion. The contents of induced DNA damage were not by antioxidant enzymes such as SOD, catalase and glutathione peroxidase or sulfhydryl groups, but a substance which is more stable in high temperature.