Abstract: The electrochemical behavior of a typical H type mediatorless microbial fuel cell (MFC), which was inoculated by the effluent of a primary sedimentation tank using potassium ferricyanide as catholyte, in the start up phase was investigated. The results showed that the change of open circuit voltage proceeded the closed one in a start up phase which revealed that the change of the internal resistance determined the start up process. The reduction of potassium ferricyanide cathode potential was smaller than the anode potential, and the changes of MFC voltage was dominated by the anode. The analysis of electrochemical impedance spectroscopy (EIS) indicated that the charge transfer resistance reduced in the start up phase, which reflected the reproduction and domestication of electricigens in MFC anode.