Nowadays, due to the complexity of the systems and the competitive conditions in the markets and industries, it is essential toreduceproductioncostsand having systems which are capable of being used in sensitive conditions. This task leads to the fact that reliability and availability concepts become more important than before.The reliability and availability are among the most important characteristics in complex systems, especially in electronic and mechanical systems of avionics, internet networks, telecommunication systems, power generation systems, weapons, etc.In this dissertation, the multi-objective availability optimization for a series-parallel system by using the redundancy allocation and availability-redundancy allocation problems are investigated. The main contribution of this study compared to the previous studies, is in the assumption of component types. Unlike the previous studies which assumed that all components are either repairable or non-repairable, in this study it is assumed that both repairable and non-repairable components are used simultaneously in the system,. Moreover, the effect of using dissimilar components in subsystems is compared with using only the same components in subsystems. In order to solve the mathematical models the NSGA-II algorithms as one of the best multi-objective evolutionary algorithms is used.