Restructuring has changed purpose of generation expansion planning (GEP) from being cost-minimization to proft-maximization. In this paper, we introduce a new formulation for objective function of generating companies (GENCOs) GEP problem in pool electricity market which includes the revenues of energy and capacity reserve markets and costs of fuel, investment, O&M, outage and emission tax. Moreover, in order to solve GEP problem with above objective function, an algorithm are introduced that use game theory and genetic algorithm for market modeling and optimization of GENCOs objective functions, respectively. To calculate the generation levels of generating units and long-term market price, we have used the traditional probabilistic production costing (PPC) which is modifed to be used in competitive electricity market.