Water management in a proton exchange membrane (PEM) fuel cell stack has been a challenging issue on the road to commercialization. This paper presents a numerical investigation of air-water flow in micro-parallel-channels with PEM fuel cell stack inlet and outlet manifolds for the cathode, using a commercial Computational Fluid Dynamics (CID) software package FLUENT. Different air-water flow behaviours inside the straight micro-parallel-channels with inlet and outlet manifolds were simulated and discussed. The results showed that excessive and unevenly distributed water in different single PEM fuel cells could cause blockage of airflow or uneven distribution of air along the different flow channels. It is found that for a design with straight-channels, water in the outflow manifold could be easily blocked by air/water streams from the gas flow channels; the airflow could be severely blocked even if there was only a small amount of water in the gas flow channels. Some important suggestions were made to achieve a better design. (c) 2006 Published by Elsevier B.V.