The superoxide scavenging activities of 12 flavonoids were measured. The superoxide anions were generated by a hypoxanthine-xanthine oxidase system and measured by the nitrite method. The results showed that the scavenging ability enhanced with an increasing number of hydroxyl groups in ring B. Substitution at C3 position with a hydroxyl group increased the activity. Compared to a methoxyl group or a glycoside in this position, a free hydroxyl group showed the highest activity. A saturated C2-C3 bond showed a higher activity than a unsaturated bond. The absence of a carbonyl group at C4 position increased the activity.