Network coding provides a powerful mechanism to improve performance of wireless networks. In this paper, we present a service curve model for performance analysis in wireless network coding. Prior work on wireless network coding mainly focuses on the overall throughput of the network. Our research aims to analyze the performance of each flow in the network. The theoretical basis of our research is network calculus. In order to model network coding in network calculus, we propose a flow-oriented definition of coding opportunity. Based on the definition, we derive formulations for computing the delay and backlog of flows in the network. Numerical results show that network coding can dramatically improve the per-flow performance under heavy traffic load. However, under light traffic load, it makes tiny improvement that may unfortunately be counteracted by the overhead of coding. To guarantee performance gain, we propose the concept of coding enabled area that can help to indicate when to enable the function of network coding in practice.