The oxidative stability of conjugated linoleic acid (CLA) and its ethyl ester (ethyl CLA) was compared with that of linoleic acid (LA) and ethyl LA in the bulk phase at 50°C. Progress of oxidation was determined by the measurement of oxygen absorption. The oxidative stability of CLA was almost the same as that of LA, but ethyl CLA was oxidatively more stable than ethyl LA. In the present study oxidative stabilities of soybean-triacylglycerol (TG) and TG containing CLA (CLA-TG) ware also compared. Although the content of CLA (59.4%) in CLA-TG was slightly higher than that of LA (55.3%) in soybean-TG, CLA-TG was much more stable to oxidation than soybean-TG. On the other hand, the oxidative stability of α-linolenic acid (LN) and docosahexaenoic acid (DHA) decreased by the isomerization of double bonds in both fatty acids. The same result was obtained in the case of their ethyl esters. The isomerized LN and DHA contained conjugated trienes other than conjugated dienes. The lower oxidative stability of isomerized LN and DHA would be due to conjugated trienes in these PUFAs, whereas conjugated deines had little effect on their oxidative stabilities.