The superoxide scavenging effects of fifteen coumarins were tested on the xanthine-xanthine oxidase-cytochrome C system. The results showed that fraxetin(10) displayed the strongest activity, and its percent inhibition at 100, 10 and 1 μM were 100, 100 and 53.13% respectively. Esculetin(4) showed the second strongest activity resulting in percent inhibition at 100 and 10 μM were 87.16 and 52.38% respectively. Both fraxetin(10) and esculetin(4) have been isolated from the plant, Fraxinus bungeana DC (Oleaceae) which has been used in folk medicine as an analgesic and anti-inflammatory medicine. It seems that two phenolic hydroxy groups in the ortho position in the molecule of coumarins play an important role in scavenging activity.