The aging of machine can be slowed and the life of machine can be increased by implementing periodic predictive maintenance. In this article,an improvement factor considering maintenance cost,age of machine and learning effect in the process of maintaining is constructed to describe the effect of periodic preventive maintenance. The model of improvement factor can be fitted through health index. Based on improvement factor with infinite time,cost function including preventive maintenance cost,minimal repair cost and replacement cost is explored. The unit cost and the optimized maintenance time interval based on limit of reliability are deduced. The process of optimization and an example are given.
