The antioxidant and synergistic effect of methanol extracts of onion skin were investigated by measuring peroxide value. The inhibitory effects of the extracts against metal catalyzed oxidation were also studied. The antioxidant activity of methanol extract (0.02%) of onion skin was stronger than that of mixed tocopherol or ascorbic acid (0.02%), but weaker than that of BHT (0.02%). However, the methanol extract at the concentration of 0.04% exhibited strong antioxidant effect, comparable to that of BHT (0.02%). The methanol extract showed very strong synergistic effect with the mixed tocopherol. The methanol extracts at the concentration of 0.03% and 0.04% acted as metal deactivator in the presence of FeC1$_3$. As a metal deactivator, methanol extract of onion skin appeared to be more effective than citric acid.