Six alkaloid compounds were isolated from the chloroform soluble fraction of the methanolic extract of the wood of Picrasma quassioides Benn (Simarobaceae) as the cytotoxic components against the tumor cell lines, U937 and HepG2. The compounds were identified as 4,9-dimethoxy-1-vinyl- $eta$-carboline (1), 1-carbomethoxy- $eta$-carboline (2), 4,5-dimethoxycanthin-6-one (3), 5-hydroxy-4-methoxycanthin-6-one (4), 3-methoxycanthin-5,6-dione (5) and 4,8-dimethoxy-1-vinyl- $eta$-carboline (6) by spectroscopic analysis. Among them, compound 5 showed the most potent cytotoxicity in a dose-dependent manner against the two tumor cell lines. This is the first report of the cytotoxic effect of those isolated compounds 1-5 on U937 and HepG2 cell lines.