Maintenance scheduling of generating units in restructured power systems is a collaborative and interactive process between independent system operators (ISOs) and generating companies (GENCOs). The ISO should comply with GENCO maintenance preferences subject to targeted system reliability levels. This process might be multistage since the admission of all initial unit outage requests may threaten system reliability. Hence, the ISO accepts some proposals and determines alternatives for the remaining units. The GENCOs are then allowed to confirm the alternatives or revise them. In this paper the ISO problem of generating unit maintenance scheduling is tackled. The objective function is to minimize the deviation of awarded schedule from the requested schedule, as measured in MW weeks. A novel and effective optimization technique is developed to solve the problem at hand. In the proposed methodology, risk leveling and dynamic programming optimization algorithms are concurrently utilized to lessen the computation burden and enhance the solution process. The effectiveness of the proposed method and its applicability to real-life systems are verified by examining the generation section of the IEEE reliability test system (IEEE-RTS). The performance of the new method is also compared with that of the individual risk leveling approach