In this paper, we present a reliability growth model to describe the changes of failure rate at several stages of the development testing program. At stage k, Bayes method is used to get the failure rate of system, then the result is updated according to the duty cycle. The prior distribution of λ_k is developed to use the information of test 1to k-1. A growth factor η_k is introduced at each stage to show the reliability growth. A numerical example illustrates the use of the model.