A reactive oxygen species has been implicated in a range of human pathological diseases such as atherosclerosis and certain cancers. Flavonoids are reported to exhibit various biological activities, including antioxidative and free radical scavenging activities. Several flavonoids obtained from barley leaves, soybean and some medicinal plants, Silybum marianum, Sophorae Flos, Cinnamon, Ephedrae Herba and Scutellariae Radix, were tested for their DPPH (1,1-diphenyl-2-picrylhydrazyl) radical scavenging activity. The structure-activity relationships suggested that not only the numbers of hydroxy group but also the position of hydroxy group might be important for mediating potent activity.