The effects of operational conditions of a microbial fuel cell were investigated and optimized for the best performance of a mediator-less microbial fuel cell by Gil et al.6. The optimal pH reported was 7. The resistance higher than 500 Ω was the rate-determining factor by limiting electron flow from anode to cathode. At the resistance lower than 200 Ω, proton and oxygen supplies to the cathode were limited. For the construction of an efficient microbial fuel cell, a non-compartmentalized fuel cell with an electrode having a high oxygen reducing activity should be developed. Since the concentration of fuel determines the amount of electricity generation from the fuel cell, the device can be used as a BOD sensor.