Over the last decade, high-pressure treatment has been of considerable interest as an alternative to thermal treatment for food preservation and processing. The impact of high-pressure treatment on lipid oxidation in fish oil-in-water emulsions stabilised by 0.5 wt% whey protein isolate or sodium caseinate was investigated by determining thiobarbituric acid (TBA), propanal values and hydroperoxide values (PVs). The TBA value and the PV of all emulsions increased with increasing pressure at low temperature, indicating that lipid oxidation was promoted by high-pressure treatment. The impact of high-pressure treatment on the oxidative stability of lipids was increased when the temperature was increased as the TBA and propanal values were markedly enhanced by high pressure at high temperature. However, high-pressure treatment did not affect the antioxidant properties of whey protein isolate and sodium caseinate in the fish oil-in-water emulsions, which may suggest that high-pressure treatment does not alter the lipid oxidation pathway in emulsion systems. The promotion of lipid oxidation by high pressure is due mainly to increasing the pressure on a gas reaction shifts the position of equilibrium towards the side with fewer gas molecules.