The system approach to the problem of preventive maintenance scheduling for thermal units in a large scale electric power system is considered in this paper. The maintenance scheduling program determines a set of thermal units maintenance switch off for a time period of one year. This paper considers the application of dynamic programming and successive approximations method in determination of annual thermal unit maintenance schedules. The objective function is multiple component and consists of system operation costs and system reliability indices (loss-of-load-probability and expected unserved energy). The evaluation of these costs is performed through a simulation method which uses a cumulant load model. The software package, developed in FORTRAN and integrated with an ORACLE data base, produces many useful outputs.