Based on reliability theory statistical survey of the fault occurrence of the home-made truck of 5 ton pay load was conducted. The experiment was carried out in real operation conditions. Actual breakdown records of the main parts are taken down, which provides a scientific basis for the reform of the truck maintenance system. The effect of the quality of engine oil on the wear of engine parts was tested and analyzed, and reasonable intervals of oil change was calculated. A new maintenance system for the said truck was then specified on the basis of cost-benefit analysis, ‘conduct periodical tests and maintenances with the prevention of trouble first’ is taken as the principle. The social and economic benefits are obvious. The experiment also provides a package of test and calculation procedures for optimizing the maintenance intervals of other type of motor vehicles.