Intrusion tolerance is the rising third generation technology of network security. For the shortcomings of existing models, a model of intrusion tolerant system based on game theory is proposed. The intrusion tolerant system and the intruder are seen as the two sides of the game. The income functions of the two sides are designed basing on some given concepts. Through quantifying and analyzing the income functions, the optimum strategies of the intrusion tolerant system and the intruder are obtained, and the Nash equilibrium of the game system is achieved finally. The results of analysis show that, this proposed model of intrusion tolerant system is consistent with the practical system.