In this paper, we focus on the availability of software systems. Software systems with long-running execution may lead to software aging. This phenomenon affects the performance of software system and may eventually cause them to crash or failures. To counteract the phenomenon aging, software rejuvenation is implemented to prevent severe software failures. Generally, when software is initiated, amounts of memory are allocated. Then the body of software is executed for providing a service and when the software is terminated, the allocated memory is released. We propose a software rejuvenation model based on degraded physical memory. This model is implemented with a Markov chain and the system performance due to memory usage, by services, divided into four equal level. Hence, we offer four types of policies for software rejuvenation. The goal of the proposed method is to optimize availability of system. The results we achieve show the superiority of proposed method. تفاصيل المقالة