Canthin-6-one (I), canthin-6-one 3-oxide (III) and new alkaloid, 1-methoxycanthin-6-one (II) were isolated from the wood of Ailanthus altissima Swingle (Simaroubaceae). The structure of II was elucidated by spectroscopic studies and chemical evidences. © 1976, The Pharmaceutical Society of Japan. All rights reserved.