Tetracyclic triterpenoids (named as altissimanins A-E, 1-5) and a terpenylated coumarin (denominated as altissimacoumarin G, 6), along with fifteen known compounds (7-21) were isolated from the bark of Ailanthus altissima. Structures of compounds 1-6 were established by spectroscopic methods and chemical transformations. Altissimanin A (1) is a tirucallane-type triterpenoid bearing an uncommon oxetane ring in the side-chain, while altissimanins D (4) and E (5) are two unprecedented dimers each consisting of one tirucallane-type and one dammarane-type triterpenoid moiety. All the isolates were evaluated for their cytotoxic effects against a small panel of human cancer cell lines.