Abstract:A new triterpene saponin B has been isolated from the earial parts of Polygala japonica Houtt in folk-lore medicine. Its molecular: C48H78O20, m.p. 199–202℃, [α]D23+30.0 (C, 0.5, CH3OH). Acidic hydrolysis of this saponin gave a sapogenin (2α, 3α, 24-tri-hydroxyolean-12-ene-28-oic acid) and D-glucose. The structure of saponin B was elucidated as 28-O- [β-D-glucopyranosyl (1→2) -β-D-glucopyranosyl (1→2) -β-D-glucopy- ranosyl] 2α, 3α, 24-trihydroxyolean-12-ene-28-oic acid mainly by 13C-NMR, MS and some chemical transfomations.