Oxygen transport to the reaction sites through electrode structure is crucial to the operation and performance of polymer electrolyte membrane fuel cells. In this study, a 3-D steady-state simulation is developed to investigate the oxygen transport phenomena utilizing the measured ex-situ transport properties and the actual dimensions of the cell components as input parameters, and the simulation is validated with the in-situ experimental data on the current-voltage relation. The pore structure is measured by the method of Standard Porosimetry based on capillary equilibrium. The indicators for the ability of mass convection and diffusion in the electrodes, permeability and effective diffusibility, are measured based on the Darcy's and Fick's laws, respectively. The simulation results demonstrate that the average through-plane convective oxygen flux is about 20% of its diffusive counterpart under small current densities (0.10 A.cm(-2)), while it is reduced to only 5-14% under high current densities (1.22 A.cm(-2)). The local oxygen transport also varies significantly with the locations inside the cathode electrode under various average current densities. The results suggest that if measures can be taken to enhance the convection in the porous electrode under high current densities, the transport of oxygen to the reaction sites can be considerably improved.