P2P systems have suffered a lot from free riding problem i.e. some participants consume more resources than they contribute. In order to deal with this problem efficiently, an incentive model based on peer credit has been proposed. Each peer in this model is a credit entity. Through introducing reward method to the model, peers will allocate resources according to the credits of requesting peers to maximize their own credits. A backtracking algorithm has been suggested to solve the issue of maximizing credit rewards. To avoid inflation of the credit, attenuation method is introduced. The scheme how to apply the credit model in hybrid P2P networks has also been elaborated in this work. The experimental results show that the model can effectively control free riding and improve efficiency of the system.