This study was aimed to evaluate the relative antioxidant activities of some common flavonoids such as quercetin, rutin, catechin, epicatechin gallate (ECG) and epigallocatechin gallate (EGCG). The antioxidant activities were assayed by the inhibiting capacity of oxygen consumption and lipid peroxidation and their relative activities were compared to that of Trolox , an aqueous derivative of a-tocopherol. Oxygen free radicals were produced by several systems including AAPH generating peroxyl radicals, metal-catalyzed oxidation (MCO) system (ascorbate-Fe2+), and hypoxanthine-xanthine oxidase (HP-XO) system. EGCG was identified as the most potent antioxidant against AAPH as well as MCO system-induced oxidative damage. Interestingly, all three catechins showed especially potent antioxidant activity against MCO system suggesting the possible antioxidative mechanism of metal ion-chelation. However, all flavonoids tested except quercetin showed little antioxidant activity against HP-XO system-induced oxygen consumption. The inhibition of both AAPH and MCO system-induced lipid peroxidation of liposomes by flavonoids took on a similar pattern to the inhibition of oxygen consumption, and EGCG was the most effective antioxidant against lipid peroxidation. The antioxidant activity against protein oxidation was not investigated due to the direct inhibition of glutamine synthetase activity by some flavonoids. These results showed that some flavanols such as ECG and EGCG scavenged oxygen free radicals and inhibited lipid peroxidation of liposomes effectively induced by either AAPH or MCO system. Especially, EGCG was the most potent antioxidant among tested flavonoids.