A 3D micro-scale model is developed to simulate the transport and electrochemical reaction in a composite cathode. This model takes into account the details of the specific cathode microstructure such as random pore structure, active TPB (three phase boundary) site distribution, particle size and composition and their interrelationship to the charge transfer and mass transport processes. Especially, the pore structure and mass diffusion were incorporated into this model. Influence of the microsturcture parameters on the performance was investigated by numerical simulations. Simulation shows that the cathode porosity should be in the range of 0.25-0.45 for the optimized performance. A larger thickness is in favor of increasing the effective reaction sites and reducing total specific resistance. However, the thickness needs to be confined below a certain thickness in order to prevent lower utilization and excessive concentration loss. The fine particles contribute to increase the TPB length and thus decrease the activation overpotential. On the other hand, the extremely small particles might lead to an excessive diffusion resistance. The model was validated based on the continuum model and experimental data to determine its accuracy in predicting the cathode performance. The results generally agree with the experimental data from the literature. However, as compared with the continuum model, the predicted total resistance is slightly higher for a thin electrode. (c) 2007 Elsevier B.V. All rights reserved.