ABSTRACTFresh mackerel fillets with skin on were pretreated, before frozen storage at –15°C by microwave heating at 2450 MHz for 5, 10 and 20 sec. During 6 months, the lipids of the skin and meat of the fillets were separately examined for quality variations by chemical methods such as TBA, POV and FFA. Pro‐oxidant effects on lipids in mackerel fillets pretreated for 20 sec were shown by an increase of TBA molar value of 200% and 100% in the skin and meats, respectively, after 6 wk of frozen storage at –15°C. In fillets stored for 6 months, the rate of increase in POV in the pretreated sample was twice that in the control. According to the RIM quality evaluation system, the life in frozen storage of mackerel fillets pretreated with microwave heating decreased proportionately with the time of pretreatment. The loss of storage life was as much as 60% (from 16 to 7 wk) compared with the control samples. It is suggested that the promotion of lipid oxidation in the fish pretreated with microwave energy should be considered as a particularly important factor affecting the quality during frozen prese