A three-dimensional, two-phase, multiple-component, unsteady model for mass transport at the cathode side of a passive direct methanol fuel cell was developed. We assumed that the cathode was full of dry air at the beginning. The air diffuses through the cathode diffusion layer to the cathode catalyst layer where electrochemical reaction occurred. The liquid water which generated in the cathode catalyst layer is gradually moved out of the cathode catalyst layer by diffusion and gas carrying. The numerical results indicate that at the constant current density of 100 mA/cm2, the quantity of liquid water increases firstly and then keeps in a constant in both cathode diffusion layer and cathode catalyst layer while that continually rises in the air cavity. This phenomenon is caused by the gravitational effect. Under normal gravity, the liquid water flows down in the cathode catalyst layer and accumulates at the bottom. When the gravitational effect is greater than the viscous forces, the liquid water droplets escape from the cathode diffusion layer and then fall into air cavity. The distributions of species concentration and velocities along different directions are also presented.