This paper deals with modeling the operational reliability of a bus fleet. The operational reliability is represented by the rate of occurrence of minor failures. A power-law failure point process model with the cumulative numbers of type-II and type-III preventive maintenance (PM) as covariates is fitted to a set of real-world data. We evaluate the effect of these two types of PM on minor failures of buses using the fitted model. This provides a method to quantitatively evaluate the effectiveness of a kind of PM actions.