An optimum electrode spacing of less than 1cm was determined for an air cathode microbial fuel cell (MFC) with a membrane electrode assembly (MEA) system. The lag period decreased as the electrode spacing increased and the voltage generation increased. Stable power density increased from 93 mW/m(2) to 248 mW/m(2) when the electrode distance increased from 0mm to 9 mm. In the polarization test, a maximum power density (400 mW/m(2)) was obtained at a distance of 6mm. The oxygen mass transfer coefficient (KO=4.60×10(-5) cm/s) with a 0mm spacing was five times higher than that at a 9 mm spacing (0.89×10(-5) cm/s). Long-term operation of the MFC exhibited relatively stable anode potentials of -285±25 (0 mm) and -517±20 mV (3, 6, and 9 mm) and a gradual decrease in cathode potential for all distances, especially with 0-mm spacing. The performance of air cathode MFCs can be improved using minimum electrode spacing rather than no spacing.