In this paper, we study a maintenance model with a given type of service contract. In order to study the optimal designing of machine maintenance contracts, this paper develops a multi-server queueing model to obtain the expected net incomes by each machine and the agent. Using a non-cooperative game formulation, we achieve the agent's optimal pricing strategy, the length of warranty and the number of repairmen. We find that the optimal length of warranty and the number of repairmen are independent of the warranty price functions. Furthermore, for fixed lifetime of the unit, the number of customers hardly affects the optimal length of warranty.