To investigate the antioxidant activity of mustard leaf (Brassica juncea), we prepared four fractions (CH(2)Cl(2), EtOAc, BuOH and H(2)O fractions) and examined their radical scavenging activities in vitro and in vivo. Based on the in vitro results of spin trapping and 1,1-diphenyl-2-picrylhydrazyl radical, we carried out an in vivo study with the BuOH fraction to investigate its effect on oxidative stress in rats with streptozotocin-induced diabetes. We found that in comparison with untreated diabetic control rats, oral administration of the BuOH fraction (100 or 200 mg/kg body weight/day for 10 days) induced a significant decrease in serum glucose and glycosylated protein, which is glycosylated with hemoglobin as an indicator of oxidative stress. Moreover, administration of the BuOH fraction also effectively reduced the serum superoxide and nitrite/nitrate levels. Furthermore, the levels of thiobarbituric acid-reactive substances in serum and liver were also significantly lower than in the control group. These results indicate that the BuOH fraction of mustard leaf controls glucose metabolism and reduces lipid peroxidation as well as the level of oxygen radicals, ameliorating the damage caused by oxidative stress in diabetes.