Mutagenic acivity of essential oils was tested using Salmonella typhimurium TA100 in the presence or absence of 59 fraction prepared from the mouse liver. Growth inhibitory effect of the oils on the bacteria was measured to warrant the mutagenic effect. Most oils were (round to be very strongly toxic against the bacteria at a high dose $(2,000{mu}g/plate)$. At lower doses than this concentration, the Curcuma longa oil was found to be the most mutagenic with S9 fraction whereas it was not mutagenic without the fraction suggesting that this oil could undergo activation for the mutagenicity by cytochrome P45O. However, the mutagenicity of the Eugenia caryohpylata oil was disappeared under S9 fraction. Other oils obtained from Cinnamomum cassia, Chrysanthemum sibiricum, Paeonia moutan the flower of Artemisia princeps var. Orientalis, Allium sativum, were not mutagenic. This result suggested that antimutagenicity assay on the essential oil is necessary for the biological available substances.