Abstract: Very few studies have addressed the problem of modeling and evaluating the reliability growth of combined HW/SW system, A new modeling method is presented in this paper to treat those problems, which is based on two main assumptions: 1) The failure rates are constant during periods between failures, while changes on hardware (software) failure rates only occur in each hard- ware (software) failure recovered points. 2) More than 1 failures occur at one time is impossible. Analytic expressions of system availability indexes are also presented, as well as computing complexity. An approximate method is given via field data example to reduce the complexity.