Three new limonoid-type triterpenoids, namely toonasins A-C (1-3) with a rare lactam E ring, along with six known compounds (4-9) were isolated from the barks of Toona sinensis. The structures of new compounds were elucidated by interpretation of spectroscopic data, and the relative configuration of compound 1 was further characterized by X-ray crystallographic analyses. The isolated compounds were evaluated for their cytotoxic activities against five human tumor cell lines (HL-60, SMMC-7721, A-549, MCF-7 and SW480), and compounds 3 and 5 showed weak cytotoxicities.