The essential oil (EC-oil) obtained from the buds of Eugenia caryophyllata (Myrtaceae) was examined for its free radical-scavenging activity, cytotoxicity, and in vivo toxicity. To find the xenobiotic properties of EC-oil, serum thiobarbituric acid reactive substances (TBARS) level and hepatic drug-metabolizing enzyme activities were measured. It was found that EC-oil displayed xenobiotic properties like bromobenzene. The cytotoxicities of eugenol and of the EC-oil were greatly attenuated by the sulfhydryl-containing N-acetyl-L-cysteine (NAC), suggesting that eugenol was susceptible to nucleophilic sulfhydryl. In addition, eugenol also showed potent free radical-scavenging activity in the 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. Moreover, methyleugenol considerably exhibited less cytotoxicity and less potent free radical-scavenging activity than eugenol, and the cell viability of the methyleugenol was more increased with NAC treatment than the eugenol. These results indicate that the phenolic OH in eugenol may play a crucial role in both cytotoxicity and free radical-scavenging activity. The fashion on oxidative stress and hepatic drug-metabolizing enzyme activities of eugenol resembled those of bromobenznene.