To create tribosystems that are environmentally-friendly and free from health hazards, many researchers and engineers have tried to apply vegetable oils as practical lubricants. Our preliminary findings have shown that rapeseed oil and soybean oil exhibit a low friction coefficient and superior wear durability when used for the combination of a bearing steel ball and a phosphor bronze plate under temperature conditions of 40°C and 100°C. However, the long-term performance of tribosystems using vegetable oils has not yet been fully clarified. In our research, we have used an accelerated test method to investigate the transition of lubricity of a rapeseed oil related to its oxidative deterioration. The wear durability of the combination of a bearing steel ball and a phosphor bronze plate was improved together with the increased acid number (AN) of the rapeseed oil at both 40°C and 100°C. On the other hand, though the combination of a bearing steel ball and a bearing steel plate showed the same tendency at 40°C, the tendency was reversed at 100°C. It may have been induced by differences between the bearing steel and the phosphor bronze on both adsorptive capacity with fatty acid and corrosion-resistant characteristics.