A three dimensional (CFD) model is applied to investigate the effect of geometry designs and dimensions of the flow field on the performance of PEM fuel cells with an area of 5.1 x 5.1 cm(2) and composed of 25 channels. Three designs including serpentine, interdigitated and parallel with six channel to rib width ratios defined as (eta = channel width/rib width) have been analyzed. In order to keep the area of the cell and number of channels constant, when the width of the channel increases the width of the ribs decreases and vice versa for a comparative purpose. At high operating voltage, the geometric designs and channel to rib width ratios have little influence on cell performance while at low operating voltage, it affects considerably the cell performance. The PEM fuel cell with serpentine designs has better performance. It increases by 4.6% and 39.1% compared to interdigitated and parallel, respectively. Moreover, decreasing channel width and increasing rib width improve cell performance. It increases up to 120% for serpentine, 45% for interdigitated and 23% for parallel when channel to rib width ratio decreases from 2.66 to 0.25. The local transport phenomena have been also analyzed. The pressure drop and under rib convection increase with decreasing channel to rib width ratio, which results in high reactants speed under the ribs and more homogenous local transport phenomena in the catalysts layer; enhancing the net power density.