A three-dimensional, steady-state, nonisothermal gas-liquid two-phase mathematical model for the proton exchange membrane fuel cell (PEMFC) is developed to better investigate transport phenomena and electrochemical kinetics in the whole fuel cell with straight channel and to predict the performance of fuel cell. Almost all transport phenomena occurring in the fuel cell such as fluid flows, heat transfer, mass transfer, electrochemical kinetics, and charge transport are accounted for in this comprehensive model. Using a multiphase mixture model to describe two-phase flow and transport phenomena, this model can handle the situation where a single-phase region coexists with a two-phase zone in PEMFC easily. An important feature of this model is that it can model and predict electrochemical kinetics in detail (i.e., transport of electrons in the carbon phase), and protons in the membrane phase are accounted for spatially. Three-dimensional spatial distributions of temperature, species concentrations, and carbon-phase and membrane-phase potentials are illustrated and discussed in detail by numerical simulation. Furthermore, comparison of predicted performance and experimental data published in the literature is conducted to verify the mathematical model. Last, effects of velocity and humidification temperature of the cathode inlet stream and shoulder-to-channel ratio on the cell performance have been analyzed.