A multistate system (MSS) with independent components having many levels of degradation has been considered in this article. Periodic inspection and maintenance have been performed for each component. For evaluating steady state probabilities of each component, the components have been modeled as discrete state continuous time semi-Markov process. Steady state reliability indices of availability and performance deficiency are obtained using UGF technique to avoid computational complexity of random process method. Analytical procedures have been illustrated based on a real data from a typical MSS of a power station with independent generators where in we showed how the availability of the whole system has been enhanced through periodic inspection and maintenance.