Chemiluminescence (CL) analysis was used to evaluate the thermal deterioration of hard milk fat two pressurization periods. Hard milk fat was heated at 200°C for 1 h, 2 h or 3 h after high-pressure treatment (400MPa) at 45°C for 1 h or 4 h. The CL intensity of hard milk fat was then measured at 45°C, 50°C and 55°C and increased with heating time at each temperature. When the intensity was measured at 50°C or 55°C, the effect of pressurization time was difficult to discern, because the intensity varied widely during measurement. In contrast, at 45°C, the CL intensity of heated hard milk fat that had been pressurized for 4 h was significantly lower than those measured at other temperatures for each heating period. The intensity of non-pressurized hard milk fat was observed to be significantly higher than that measured at other temperatures for each heating period, indicating that pressurization for 4 h yields the highest suppression of the thermal deterioration of hard milk fat.