Caragana tibetica KOM. (Fabaceae) is a medicinal plant that has been traditionally used in western part of China. In the course of our screening study on antioxidant activity of medicinal plants, the 70% acetone extract of the stems of C. tibetica was found to have a potent superoxide anion scavenging activity. Tibeticanol (1), a new piceatannol dimer possessing antioxidant activity, was isolated along with eleven known aromatic compounds. Their structures were elucidated on the basis of NMR and MS data. Enzyme oxidation of monomeric stilbene, piceatannol (3), with horseradish peroxidase and hydrogen peroxide yielded cassigarol E (5) and G (6) as major products. Most of the isolated compounds exhibited superoxide anion scavenging activity.