We have undertaken four basic in vitro studies and an animal experiment to obtain information about the antioxidant activities of buckwheat hull extract (BWHE). In the in vitro studies, BWHE scavenged super oxide anion produced in the xanthine/xanthine oxidase system (IC50=11.4 μg phenolic compound/ml), and strongly inhibited autoxidation of linoleic acid (IC50=6.2 μg phenolic compound/ml). Low-density lipoprotein (LDL) oxidation induced by Cu2+ ion was also protected by BWHE. In the animal experiment, ddY mice were fed a standard diet supplemented with 0.75% BWHE for 14 d. In blood, liver and brain of the mice TBARS and fluorescent substance concentration were significantly decreased compared with those of non-treated mice. SOD like activity in serum also significantly rose by BWHE treatment. BWHE was shown to be effective for protecting biological systems against various oxidative stresses in vitro, and to have antioxidant activity in vivo.