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Continuing our studies on bioactive constituents of Rubus aleaefolius Poir. (family Rosacae) [1,2], we report here a new pentacyclic triterpene ester, 3b-O-trans-ferulyl-2a,19a-dihydroxy-urs-12-en- 28-oic acid (1) from Rubus aleaefolius Poir., which inhibits mammalian cell cycle at G2/M phase. The roots (3 kg) of R. aleaefolius were extracted with 60 % aqueous alcohol to give an alcoholic extract (435 g) possessing potential inhibitory activity on cell cycle of tsFT210 cells [1]. The alcoholic extract was thus suspended in water and then extracted successively with chloroform and ethyl acetate to obtain an active ethyl acetate extract (78 g). This extract was further separated by repeated solvent-extraction, silica gel column chromatography, preparative HPLC and recrystallization procedure in a bioassay-guided manner to obtain pure 1 (26.4 mg). Compound 1, white crystal powder (from MeOH), mp 232-234°C, [a] 25 D +1.6 ° (c 0.23, MeOH), gave positive reaction to the Libermann-Burchard and FeCl3 reagents, showing typical colors for triterpenoid and phenol, respectively, and its molecular formula, C40H56O8 (molecular weight 664), was determined by positive HR-ESI-MS measurement. It showed UV absorptions at 233 nm (log e 4.05), 299 sh (4.11) and 325 (4.25) ascribable to an a,b-unsaturated aromatic moiety and the IR spectrum indicated the presenc