The essential oils from leaves of three Cinnamomum osmophloeum clones (A, B and C) and their major chemical constituents were investigated. GC and GC-MS analyses showed that Cinnamomum osmophloeum clones A, B and C contain trans-cinnamaldehyde (90.6%), eugenol (91.3%) and linalool (83.7%), respectively, as the major component. The inhibitory effects of the essential oils on the growth of human leukemia cell lines U937 and K562 Hep-1, in vitro were investigated. A strong cell proliferation inhibition activity for Cinnamomum osmophloeum clone A was observed for cell lines K562, whereas Cinnamomum osmophloeum clone B and Cinnamomum osmophloeum clone C gave comparatively low inhibition. This trend paralleled the cytotoxic properties of authentic trans-cinnamaldehyde, eugenol and linalool, respectively.