Anti-proliferative activity of essential oil from 17 Thai medicinal plants on human mouth epidermal carcinoma (KB) and murine leukemia (P388) cell lines using MTT assay were investigated. An amount of 1 x 10(4)cells/well of KB cell line and 1 x 10(5) cells/well of P388 cell line were treated with the oil samples at different concentrations ranging from 0.019 to 4.962 mg/ml. In KB cell line, Guava (Psidium guajava L.) leaf oil showed the highest anti-proliferative activity with the IC(50) value of 0.0379 mg/ml (4.37 times more potent than vincristine) whereas Sweet Basil (Ocimum basilicum L.) oil gave the highest anti-proliferative activity with the IC(50) value of 0.0362 mg/ml (12.7 times less potent than 5-FU) in P388 cell line. The results demonstrated the potential of essential oil from Thai medicinal plants for cancer treatment.