Effects of channel and rib widths with an aspect ratio (H/W) of 0.67 of rectangular cross-sections flowfield plates on cell performance in terms of VI/PI curves versus flowfield pressure drop of a micro-proton exchange membrane (PEM) fuel cell were extensively examined. The channel-to-rib width ratios of flowfield plates were varied from 0.5 to 2. It was found that a channel-to-rib width ratio of unity was secured as far as the best performance of the VI/PI curves are concerned. Moreover, the pressure drop in flowfield on both anode and cathode was also measured. With the aid of pressure drop obtained for H2/air delivery, an optimal channel-to-rib width ratio in terms of the net power gain factor (=power gain/power consumption) of 0.67 can be obtained for the cases under study. Furthermore, a simple lumped capacitance model was used to predict the temperature evolution of the fuel cell system, and results show that this model performs quite well, and the channel-to-rib width ratio seems no significant influence on the fuel cell temperature evolution of the flowfield plates.