Taking the fully synthetic engine lubricant as the research object, the performance and composition changes were studied and its decay characteristics was also explored after 10 000 km running of real vehicle. The results showed that the kinematic viscosity of lubricant decreased by 23.84%. Meanwhile, the total acid value and n-pentane insoluble matter increased up to 3.53 mg/g and 0.268%, respectively, and the contents of wear elements(i.e, Fe and Al) and pollutant element(i.e, Si) increased from < 1, < 1, 2 μg/g to 11, 20, 20 μg/g, respectively. During the decay process of lubricant, the decomposition and polymerization reactions of poly α olefin(PAO) made the composition of lubricant complicated, while the decomposition process was dominated by the decomposition reaction of molecular chain fracture that led to the decrease of kinematic viscosity. In addition, the reaction of oxidation and nitration also occurred, and the ketones, polycyclic aromatic hydrocarbons, amines and acids were generated with the additives consumption, which led to the deepening color and increasing total acid value. The change of lubricant molecular weight distribution indicated that both decomposition and polymerization reactions occured during the decay process, while the chain-breaking decomposition reaction was dominated.