Yearly preventive maintenance scheduling of generating units in a restructured power system is one of the most important problems that have to be solved in modern power systems. In this paper, a bilevel approach is used for modelling of the preventive maintenance scheduling problem. The upper level of this bilevel problem represents the revenue function of power units owned by a generation company (GENCO), whereas the lower-level problem represents the market-clearing process and is usually called the independent system operator (ISO) level. This bilevel problem is then formulated as a mathematical program with equilibrium constraints (MPEC) using the primal–dual theorem, which converts the problem into a single-level mixed-integer non-linear optimization problem that can be solved using programming software. Various case studies are conducted using the IEEE reliability test system (RTS) and the obtained results are compared.