The porous diffusion medium (DM) used in fuel cells has a complex heterogeneous structure in which both hydrophilic and hydrophobic pores coexist. The capillary flow in such a mixed-wet DM is mainly controlled by the capillary pressure and saturation relation (CPSR). In order to investigate the water transport characteristics in a passive direct methanol fuel cell (DMFC), taking into account the coexistence of the hydrophilic and hydrophobic pores in the DM, we presented the mechanisms of capillary flow in the mixed-wet DM and provided a comprehensive evaluation of the CPSRs used in various existing fuel cell studies. Then, based on a two-dimensional, two-phase, nonisothermal model for the passive DMFC, we investigated the liquid transport phenomena through the mixed-wet DM by employing an experimentally measured mixed-wet CPSR. Moreover, we compared the water transport predicted by the mixed-wet CPSR and the uniform-wet Leverett CPSR for better understanding of the liquid water transport in passive DMFCs. The results show that water transport in the passive DMFC depends greatly on the CPSR of the DM, which demonstrates an urgent need for the accurate CPSRs of the DM used in fuel cells. It is also shown that the dependence of water transport on the CPSRs can be significantly influenced by the use of a hydrophobic air filter layer at the cathode.