The essential oil Origanum onites L. and its phenolic constituent carvacrol were examined for their cytotoxic and apoptotic effects in a human hepatocellular carcinoma cells Hep-G2. WST-1 and neutral red uptake assays were performed to determine the inhibitory effects of the oil and carvacrol on the growth of the cells. Possible induction of apoptosis by Origanum oil and carvacrol was further investigated by acridine orange/ethidium bromide (AO/EB) staining. Results showed that the Ori- ganum oil and carvacrol was significantly cytotoxic and induced apoptosis in Hep-G2 cells. IC value of essential oil and carvacrol was found about 0,009% (v/v) and 500 μM, respectively. After incuba- tion of the cells with Origanum oil and carvacrol, characteristics of apoptotic morphology such as chromatin condensation, shrinkage of the cells and cytoplasmic blebbing was observed. In conclusion, both essential oil and its major constituent carvacrol significantly exhibited cytotoxic and apoptotic activities in hepatocellular carcinoma cells, indicating its potential for use as an anticancer agent.