A three-dimensional steady-state electrochemical mathematical model is developed where the mass, fluid, and thermal transport processes are considered, as well as the electrochemical reaction phenomena. The influences of the parameters of interest, which include porosity, permeability, and the thickness of the gas diffusion layer, and the inlet gas stoichiometric ratio on the performance of fuel cells are identified. By applying the Powell algorithm, the optimum values of multiple parameters are obtained while optimizing the potential of the electrolyte phase at the membrane/cathode interface at a typical value of the cell voltage. Compared with the reference case, the optimized results, such as the oxygen mole fraction and the local current density distribution, provide useful information for a better design of fuel cells.