Abstarct: The issue of evaluating market power is an important challenge in power system planning. This thesis develops an analysis baxsed on a relaxation algorithm and Nikaido–Isoda function for calculation of Nash equilibrium and evolution of market structure and performance. This is done through the development of a generation company trading game that, via Nikaido–Isoda function, simulates how plaxyers coordinate their behavior in generation to maximize their profit. In this thesis, with changing type of power plants belonging to generation company and demand elasticity, market performance is evaluated. The results for pool-baxsed markets shows, generation companies coordination is a very important factor and able to decrease competition and efficiency in market.