The present study is aimed at optimization of the geometric parameters of the proton exchange membrane (PEM) fuel cells through numerical simulation. The approach is developed by integrating a direct problem solver with an optimizer. A commercial computational fluid dynamics code is used as the direct problem solver, which is used to simulate the three-dimensional mass, momentum and species transport phenomena as well as the electron- and proton-transfer process taking place in a PEMFC. On the other hand, the simplified conjugate-gradient method (SCGM) is employed to build the optimizer, which is combined with the direct problem solver in order to seek the optimal geometric parameters, including, for example, the gas channel width fraction, the gas channel height and the thickness of the gas diffusion layer. It is found that the present approach can be applied to determine the optimal set of geometric parameters, and the search process is robust and always leads to a unique final solution regardless of the initial guess. (c) 2007 Elsevier B.V. All rights reserved.