Two-phase transport is essential for water and thermal management of passive direct methanol fuel cells (DMFCs). This paper develops a three-dimensional (3D), transient, isothermal and two-phase mixture model and investigates the mass transfer characteristic in the air-breathing cathode of a direct methanol fuel cell. The velocity and volume fraction of species inside the fuel cell along with the process of electrochemical reaction are studied. The effects of several key parameters such as porosity of porous medium, current density and cell operation pressure are investigated to discuss mass transfer characteristic in the passive DMFC cathode. The numerical simulation results indicate that cathode catalyst layer porosity has definite effects on oxygen transfer and removal of water. It is also found that an adequate current density and appropriate cathode pressure could facilitate a more quickly removal of water from the cathode to the ambient. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.