Aiming at the franchising system which consists of one franchisor and two franchisees in the same market, analyses the franchisees' free-riding behaviors and the franchisor's monitoring problems. When the franchisor doesn't monitor, the service quality provided by the two franchisees has a higher positive externality and the cost of the service quality is big enough, the franchisees' free-riding behaviors will occur which will lead to neither of them providing high quality of service. If the cost is not big, the franchisor can effectively prevent free-riding behaviors by monitoring the franchisees' behaviors .Thus improves the efficiency of entire franchising system.