Preventive maintenance scheduling of thermal generating units occupies a significant place in power system operation and expansion planning. At the same time, it is a challenging problem of multi-objective type with conflicting objectives. Many previous researches argued about the importance of operating cost, while others about the system reliability as a criterion of goodness. This thesis deals with the maintenance scheduling problem as a multi-criteria decisin making (MCDM) problem. The problem is formulated as a 0-1 integer program and a solution method based on the branch-and-bound technique is developed. Some fathoming rules suitable for the proposed model are also developed. Through a case study with real data, some implications were found. They are; (1) when a system is in the state of high reserve margin, the maintenance schedule must be focused on the cost minimization side, and (2) when the capacity of the system is insufficient, impoving the system reliability must be emphasized.