The antioxidant activity of Persicaria thunberii (Polygonaceae) was determined by measuring the radical scavenging effect on 1,1-diphenyl-2-pocrylhydrazyl (DPPH) radical. The methanol extract of thunbergii showed strong radical scavenging activity at an I $C_{50}$ concentration of 12.2$mu extrm{g}$/ml, and thus fractionated by solvent extraction. The EtOAc soluble fraction was stronger than the others, and was further purified by silica gel and Sephadex LH-20 column chromatography, Antioxidant isorhamnetin, quercetin (2), quercitrin (3) and isoquercitrin (4) were isolated from the EtOAc soluble fraction. And isorhamnetin-3-sulfate (5) was isolated from the n-BuOH fraction. Compounds 2, 3 and 4 were found to have strong antioxidative potency: