Cognitive radio technology is developed to improve the utilization of radio spectrum and to solve the problems caused by the limited availability of spectrum resources. in this paper the problem of spectrum sharing among a primary user and multiple secondary users was considered. the problem was modelled as an oligopoly market competition and by using non cooperative game the spectrum is allocated to secondary users. the nash equilibrium is found and it is considered as the solution of the game. first the static game was formulated where the secondary users have current information about other users. this might not be realistic in all cognitive radio environments. thus a dynamic game was formulated where the secondary users can improve their strategy based on observations of previous strategies. the stability of the dynamic game is analysed using local stability theory.