This paper focuses on the determining the optimal replacement interval and the corresponding minimum cost of replacement for the renewal T-53 engine. It is assumed that sample failure data of T-53 engine are drawn from the mixed population, and then parameters of the failure distributions are estimated. On the basis of the above situation, the Multi-step Weibull distributions are estimated and then the optimal replacement time of T-53 engine is determined. This paper shows that if the replacement time is reduced to 2000 hours, the 2,217won of the replacement cost per unit time is only saved but also reliability of the T-53 engine is increased.