Flavonoids are a member of antioxidants commonly occurred in nature, and they have effect to prevent the generation of harmful peroxidation for human body. In addition, they have gained person's much interest through their many biological effects as anti-inflammatory and anti-allergic action recently. Since it has been clear that the peroxide lipid may cause cancer, arteriosclerosis and diabetogenous disease, so dietary intake of naturally occurring antioxidants may be effective to prevent such disease and to promote human health. More than 5000 kinds of flavonoids have been known, and classified into various classes according to their structural complexities. We investigated and reported that antioxidant activities of quercetin and its methoxides and glycosides in the previously. Quercetin showed the highest antioxidant activity among tested compounds. The aim of the present study was to examine antioxidant activities of structurally related 12 kinds of flavone and flavonol derivatives using the same model of β-carotene-linoleic acid system that assuming to oxidation in foods. Moreover, we determined structure-activity relationships based on the number and binding patterns of OH groups. All of the tested flavonoids showed delay effect of decolorization rate compared with control by antioxidant action. The level of antioxidant activity increased depending on the numbers of OH groups when hydroxyl derivatives with 2 to 5 OH groups, but compounds with 6 OH groups conversely indicated tendency to a little decline. Antioxidant activities may be affected with binding patterns as well as the number of OH groups. Chrysin showed low activity less than 10% at the concentration of 20μmol/L, while galangin and baicalein exhibited high activities more than 70% and almost 90% at the same concentration, respectively. The OH group binding at 3-position on the C ring of galangin and pyrogallol form of A ring of baicalein seemed to contribute to each antioxidant activity. Besides, the results that luteolin exceeded 90% of activity whereas apigenin had about 15% at the same concentration indicated that the catechol form of B ring may be important contribute for the activities. These OH groups were thought to participate in electron delocalization, which related to the improvable stability of the flavonoid phenoxyl radicals. By the way, robinetin and myricetin exhibited about 70% of activities, which were a little lower than quercetin of 80% activity. It seems that catechol was more suitable than pyrogallol form, in case of contribution for antioxidant activities. It is suggested that conjugation of exceeded number of OH groups may induce the possibility of decreasing in reaction level of OH groups concerning antioxidant activity because of conformational influence. Among tested compounds, quercetin indicated the highest antioxidant activity, and it has five OH groups at (1) 5,7-position of A ring, (2) 3',4'-position of B ring and (3) 3-position of C ring. Since fisetin and kaempferol showed the same degree of the activity but luteolin was lower than them, it may be suggested that the OH group at 3-podision of C ring was most important for antioxidant activities. Table. Inhibition of thermal autoxidation by selected flavones and flavonols [table]