MEMS fuel cell system was designed and fabricated for a portable power source in the present study. The system consists of a methanol reformer, a partial oxidation (PROX) reactor, and a fuel cell. Methanol reformer is a subsystem to produce hydrogen, consisting of a preheater, vaporizing/reforming channel, and a combustor. All components were fabricated using MEMS fabrication technologies integrated with catalyst coating processes. Performance of the MEMS fuel cell system was measured with the optimal conditions of reformer/PROX. The power density was 220 mW/cm2r when the potential was 0.65 V. The performance was low compared to the result for pure hydrogen because the feed at the fuel cell included undesired CO, CO2, and N2.