A new triterpenoid named alisol O ( 1) was isolated from the rhizomes of Alisma orientalis, together with six known compounds: alisol A 24-acetate ( 2), 25-anhydroalisol A ( 3), 13 beta,17 beta-epoxyalisol A ( 4), alisol B 23-acetate ( 5), alisol F ( 6), and alisol F 24-acetate ( 7). Based on 1D and 2D-NMR data (HMQC, HMBC, COSY, ROESY), the structure of the new compound was deduced to be 11-dehydroxy-12-dehydroalisol F-24-acetate ( 1). Compounds 2 - 7 exhibited inhibitory activity in vitro on hepatitis B virus (HBV) surface antigen (HBsAg) secretion of the Hep G2.2.15 cell line with IC (50) values of 2.3, 11.0, 15.4, 14.3, 0.6 and 7.7 microM, and on HBV e antigen (HBeAg) secretion with IC (50) values of 498.1, 17.6, 41.0, 19.9, 8.5 and 5.1 microM, respectively.