In the course of screening neuraminidase inhibitors from herbal medicines, Reynoutria elliptica exhibited high inhibitory activity. Four active compounds were isolated from the ethyl acetate soluble fraction by consecutive purification using sillica gel, Sephadex LH-20 chromatography, and recrystallization. The chemical structures of these compounds were identified as 1,3,8-trihydroxy-6-methylanthraquinone (emodin) 1,8-dihydroxy-3-methoxy-6-methylanthraquinone (emodin 3-methyl ether; physcion), 1,3,8-trihydroxy-6-hydoxymethylanthraquinone (omega-hydroxyemodin), and 3,5,4'-trihydroxystilbene (trans-resvertrol) by spectral data including MS, 1H-, and 13C-NMR. The IC50 values of emodin, emodin 3-methyl ether, omega-hydroxyemodin, and trans-resvertrol were 2.81, 74.07, 10.49, and 8.77 microM, respectively. They did not inhibit other glycosidase such as glucosidase, mannosidase, and galactosidase, indicating that they were relatively specific inhibitors of neuraminidase.