To solve the problem of determining the preventive maintenance interval of production plant,target function is proposed to describe the relationship between the total downtime and the preventive maintenance(PM) interval,and a delay time model is presented.The model developed is fitted to the maintenance data of recorded failures and the number of defects identified at PM.The parameter values of the process during which defects arise and of the delay time distribution are estimated using the method of maximum likelihood.Based upon the estimated model parameters and target function,an optimal inspection interval of PM is identified.