Experiments have shown that the inlet humidity has a significant influence on the performance of a polymer electrolyte membrane (PEM) fuel cell, and theory indicates that the ionic resistivity of the electrolyte membrane is dependent on the activity of water at the membrane surface. Water flux and activities change along the flow field direction. To understand the inner flow and mass transfer processes, a numerical model is developed to predict the flow inside a single fuel cell. Detailed velocity fields, pressure profiles, and current density distributions are obtained and predictions from the full-cell model are compared with the experimental data. Predictions indicate that flow inter-linkage between side-by-side flow channels occurs through the porous diffusion layer. Results also indicate that the diffusion of hydrogen is aided by the flow toward the membrane in the anode side and diffusion of oxygen is opposed by the flow direction present in the cathode side.