An anthraquinone, emodin (1), and five flavonoids, kaempferol-3-O-beta-D-glucoside (2), quercetin (3), quercitrin (4), isoquercitrin (5), and (+)-catechin (6), were isolated from an AcOEt-soluble extract of the fruits of Rumex japonicus. Their structures were determined by spectroscopic data interpretation. All the isolates were evaluated for their potential to inhibit AGEs (advanced glycation end products) formation and AGEs cross-linking, and to break already formed AGEs cross-links.