System availability is a very meaningful measure in reliability studies and achieving a high level of availability is an important goal for system designers. For a repairable system one can improve the system availability through reducing the failure rate or increasing the repair rate of its components. In this paper a repairable series- parallel system is considered and it is assumed that the life time of each component is distributed as a Weibull distribution and the repair time of each component is exponentially distributed. Accordingly availability of the system is modeled by applying a factor for reducing the failure rates which is called the reduction method and increasing the repair rates of the components by introducing an increasing factor which is called increase method. The reduction method is used according to reducing shape parameter of the Weibull distribution and increasing the scale parameter. In addition the availability equivalence factors are derived. Furthermore to show the results of this study a numerical example is presented.
