Subamone (1), a novel Cinnamomum monoterpenoid, has been isolated from Cinnamomum subavenium (Lauraceae), and its structure was determined on the basis of spectroscopic analysis. Subamone's cytotoxic activities were evaluated against A549 (human lung cancer cell), and DU-145 and LNCaP (human prostate cancer cell lines) and its cytotoxicity was found to be significantly against LNCaP rather than DU-145 and A549 cancer cell lines.