One new flavonol glycoside, 4'-O-methylmyricitrin 3'-O-beta-D-glucopyranoside (1), one new sesquiterpene, reynoudiol (11), as well as the 12 known compounds (2-10, 12-14) quercetin 3-O-methyl ether (2), quercitrin (3), isorhamnetin 3-alpha-L-rhamnopyranoside (4), tamarixetin 3-alpha-L-rhamnopyranoside (5), myricitrin (6), 4'-O-methylmyricitrin (7), isorhamnetin 3-O-beta-D-xylopyranosyl (1-2)-O-beta- D-glucopyranoside (8), isorhamnetin 3-O-beta- D-apiofuranosyl(1-2)-O-beta- D-glucopyranoside (9), (+)-catechin (10), 7-drimene-3,11,12-triol (12), clovane-2 beta,9 alpha-diol (13), and a-cadinol (14), were isolated from the methanol extract of Reynoutria japonica roots. Based on in vitro screening of the anti-influenza activity of the isolated compounds, reynoudiol showed significantly higher activity than that of oseltamivir phosphate at the same concentration, and did not induce any detectable cytopathic effect in MDCK cells. The CC50 of reynoudiol was above 50 micro M and could inhibit influenza virus infection with an IC50 of 0.29 +/- 0.01 microM. The therapeutic index (TI) of reynoudiol against influenza infection was 172.4, and thus, this compound can be potentially used to treat oseltamivir-resistant influenza virus infection.