In this paper a proton exchange membrane micro fuel cell (PEMFC) with a passive air breathing cathode and microstructured flow fields is studied comprehensively using a numerical model and experimental characterization. The design of the flow field structures is directly dependent on the fabrication technology. An optimization of structure dimensions is possible within the design space. A model is presented, which describes the influence of the aspect ratio of the channel width and the rib width of the micro flow field for a structure were no gas diffusion layers are incorporated. The fuel cell performance is limited by electrical losses in the electrode layer for large channel widths and by reactant transport for small channel widths.