Because palm-oil biodiesel is widely used as an alternative to petrodiesel, its long-term fuel properties have become of great concern to the fuel industry. In this study, we investigated the dependence of the fuel properties of palm-oil biodiesel on its oxidative degradation. We considered storage temperature, storage time, and the addition of antioxidant as possible factors influencing oxidative instability. Palm-oil biodiesel was continuously stored in a constant-temperature water bath at 20 or 60 degrees C for 3000 h to allow for variations in oxidative degradation and fuel properties such as kinematic viscosity and acid value to be observed as the storage time elapsed. The oxidative degradation of palm-oil biodiesel was caused by the peroxidation chain mechanism, which started with a hydrogen-abstraction reaction in the methylene group (-CH2-) from polyunsaturated fatty acids, leading to the formation of hydroperoxides. In turn, the fuel properties of the biodiesel became degraded. The extent to which there was chemical reactivity in the peroxidation reaction was dominated by the level of unsaturation of the biodiesel. Moreover, a higher storage temperature and a longer storage time significantly accelerated the rate at which the peroxidation chain reaction proceeded in the palm-oil biodiesel. In contrast, the addition of a chain-breaker type antioxidant was found to be effective in reducing the lipid oxidation rate and thus retarding the fuel degradation rate of the palm-oil biodiesel. Palm-oil biodiesel to which an antioxidant was added and which was stored at a lower temperature of 20 degrees C was observed to have the highest level of oxidative stability and the lowest rate of decrease in both unsaturated fatty acids and iodine value as storage time increased. The same sample of palm-oil biodiesel was also found to have the lowest rates of increase in water content, kinematic viscosity, acid value, and peroxide value as storage time increased.