There is strong interaction between heat/mass transfer and electrode dynamics in an air-breathing proton exchange membrane fuel cell (PEMFC). To investigate the heat/mass transfer characteristics of PEMFC, their effect on oxygen transport and then on the performance of the cell, a coupled three-dimensional (3D) mathematical model has been developed for an air-breathing PEMFC using non-dimensional heat/mass transfer coefficients, and a detailed interpretation of heat/mass transfer parameters in it has been done. The full elliptic Navier Stokes and energy equations are simultaneously solved in the composite domain with commercial CFD tool STAR-CD based on the finite volume numerical method. And the model has been confirmed by experimental results. From the numerical simulation results obtained, a simple equation has been suggested on the function of the sum of Grashof number and diffusional Grashof number for the calculation of dimensionless mass transfer coefficient Sherwood number. The equation is believed to be useful for a profound understanding of the nature of oxygen transport limitation coupled with the heat/mass transfer in the air-breathing PEMFC, and further for the optimum design of the air-breathing PEMFCs. The concentration over-potential as a function of limited oxygen mass transfer rate, as well as profiles of velocity, temperature, reactant and water concentration, current density are presented and discussed to analyze the coupling problems between heat/mass transfer and electrode dynamics in an air-breathing PEMFC.