Timing system is the engine coordinate system of a vehicle. Once the timing system fails, it will not only cause engine to stop working, but also result in valve damage owing to piston impact. The timing chain is the major transmission medium of a timing system. Its wear will result in pitch elongation and chain slippage, and eventually cause failure of the chain. Therefore, life of the timing chain against wear has always been an important topic for vehicle manufacturers. However, from information collected, it seems that manufacturers do not take real driving conditions into account; and the design improvement has not been related to life of the timing chain either. In view of this, the present thesis employs elasticity theory and Archard’s wear function to describe the wear amount of a timing chain. Reliability theory is also adopted in the analysis when considering random parameters such as vehicle speed, vehicle acceleration and engine speed resulted from various kinds of drivers. From a derived equation relating these parameters to the engine power, the tension of timing chain is calculated. Accordingly, the life distribution of a timing chain measured in mileage is predicted. The proper mileage for replacement and design improvement of a timing chain are discussed as well. This thesis studies in detail a timing chain fulfilling standards of JIS and ANSI. The result indicates that after the timing chain is used for 85,000 km, its reliability is 0.949. Afterwards, the reliability drops rapidly and the risk of engine-damage increases tremendously. Therefore, the timing chain is suggested to be replaced at around this mileage. Moreover, from sensitivity study, the wear coefficient and the pitch are two most important factors that would affect the life of a timing chain. They should be considered seriously by manufacturers for design improvement.