A new approach to coordinated generation maintenance scheduling (GMS) in competitive electricity market, based on game theoretic framework, is presented. The main contribution of this paper can be focused on formulating the problem of ISO and modeling the coordination procedure. In competitive markets, the objective of generation company (genco) is to maximize the profits. The objective of ISO is to ensure reliability by increasing reserve capacity during peak period. The Genco and ISO determine their own GMS according to each objective, and then GMS of Genco is compared to that of ISO with ISO reliability criteria. If they are close enough, the GMS of Genco is determined as final GMS. Otherwise the coordination procedure is repeated until the GMS of Genco satisfies the ISO reliability criteria. Numerical results for two-Genco system are used to demonstrate the applicability of this framework for maintenance scheduling problem.