This paper reports a novel configuration of stacked microbial fuel cells (MFCs) bridged internally through an extra cation exchange membrane (CEM). The MFC stack (MFCstack), assembled from two single MFCs (MFCsingle), resulted in double voltage output and half optimal external resistance. COD removal rate was increased from 32.4% to 54.5%. The performance improvement could be attributed to the smaller internal resistance and enhanced cations transfer. A result from a half cell study further confirmed the important role of the extra CEM. This study also demonstrated MFCs where the anode and cathode were sandwiched between two CEMs possessed significantly high power outputs.