A new alkaloid, 4,5-dimethoxy-10-hydroxycanthin-6-one (1), was isolated from the stem of Picrasma quassioides Bennet (Simaroubaceae) together with four known canthin-6-one alkaloids, 8-hydroxycanthin-6-one (2), 4,5-dimethoxycanthin-6-one (3), 5-hydroxy-4-methoxycanthin-6-one (4), and 3-methylcanthin-5,6-dione (5). Their structures were elucidated on the basis of spectroscopic data. The cytotoxic activity of the canthin-6-one alkaloids was evaluated using human nasopharyngeal carcinoma (CNE2) and human liver cancer (Bel-7402) cell lines. Among these isolates, compounds 1–4 exhibited significant cytotoxic activity against CNE2 cell line.