The oxidative stabilities of long chain polyunsaturated fatty acids (PUFA) such as eicosapentaenoate (EPA) and decosahexaenoate (DHA) were compared with those of linoleate (Lo) and linolenate (Ln) under light irradiation. Ethyl esters of these polyenoic acids were oxidized in separated sealed glass cylinders with constant agitation at 5°C under a fluorescent irradiation (90 Lux). Oxidation was followed by quantifying oxygen absorption, peroxide value, TLC and high pressure gel permeation chromatography. Both EPA and DHA were rapidly oxidized without a distinct induction period, while the induction periods of Ln and Lo were 1–2 and 15 days, respectively. The relative oxygen uptakes (Lo = l) during the first two days were as follows: Ln 99; EPA 743; DHA 948. We did not detect any significant differences in the oxidation rate during the propagation stage among the esters tested. The low values of OOH-oxygen/total oxygen uptake observed in polyenoic esters with more than 3 double bonds suggest the unstability of hydroperoxide in these esters. The major secondary products in EPA and DHA were polar materials consisting mainly of dimers. The intermolecular linkage distribution of dimers of PUFA commonly showed 80% C-O-O-C and 20% C-O-C.