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.　Copyright　©　2012　by　ASME.