There are two distinct types of maintenance action, namely preventive one and corrective one. Preventive maintenance is performed at regular intervals and can contribute significantly towards the increase of reliability and availability. It must be scheduled carefully in order that the availability is maximized through optimizing regular interval. On the other hand, corrective maintenance is performed when the system fails, and so the occurrence of corrective maintenance action is a random variable that cannot be predicted beforehand. From these considerations, it is clear that time is the most important factor in maintainability, and therefore, we classify maintenance data into two groups, that is, scheduled maintenance data and unscheduled maintenance one. Next, based on these classified data, we propose the new availability model which modifies Policy II proposed by R. Barlow and L. Hunter. Finally, we show the usefulness of the new model proposed here by applying these theoretical results to real data of some power plant.