Microbial fuel cells (MFCs) use microorganisms to convert organic matter into electricity. In order to enhance the mass transfer of MFCs, four types of simplified flow channels, without flow obstacles (square, circular, divergent and convergent), were designed and applied to the anode/cathode channels of MFCs. The simulation analysis showed that the four types of simplified flow channels without flow obstacles obtained a better flow mixing efficiency with an Aspect Ratio (AR) of 1 at a Reynolds number (Re) of 60. A maximum power density of 617.8 mW/m(2) and a COD (chemical oxygen demand) degradation ratio = 9.9% were obtained by the MFCs with the convergent types of flow channels without flow obstacles. This is because the flow mechanism (convection and vortex flow) generated by convergent types of flow channels decrease the mass transfer and ohmic losses. Therefore, this concept of the simplified flow channel without flow obstacles will be useful to the application of MFCs in the future. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.