Bioassay-guided fractionation of methanol extract of Reynoutria sachalinensis flower using DPPH assay has led to the isolation of three anthraquinones and three flavonoids. Their structures were identified as emodin (1), emodin-8-O-beta-D-glucopyranoside (2), physcion-8-O-beta-D-glucopyranoside (3), quercetin-3-O-alpha-L-arabinofuranoside (4), quercetin-3-O-beta-D-galactopyranoside (5), and quercetin-3-O-beta-D-glucuronopyranoside (6) by comparing their physicochemical and spectral data with those published in literatures. All isolated compounds were evaluated for antioxidant activities with free radical 1,1-diphenyl-2-picrylhydrazyl (DPPH) scavenging, superoxide radical scavenging and Cu2+-mediated low density lipoprotein (LDL) oxidation assay. The results demonstrated that three flavonoids, 4, 5, and 6 had remarkable antioxidant activities with the IC50 values of 64.3, 54.7, and 46.2 microM (DPPH scavenging), the IC50 values of 6.0, 6.7, and 4.4 microM (superoxide radical scavenging) and the IC50 values of 3.8, 3.2, and 5.4 microM against LDL oxidation, respectively.