Abstract: Analyzing the statistical methods of probability density function of loading, the relevant transverse and longitudinal load distributions are defined with the reliability models proposed. Because the transverse distribution is usually obtained easily, the relationship between reliability of a part and loading frequency is emphatically studied when the transverse distribution is known, then a model is developed to calculate reliability. In addition, assuming that the loading process is a Poisson process, the relationship between reliability and service time is analyzed considering strength degradation and using the equivalent loading. The results show that for the same probability density function of loading there are some quite different values of reliability if using different statistical methods. Therefore, the reliability model should be chosen correspondingly according to the statistical method to be used.