Co-laminar flow microbial fuel cells (MFCs) with flow-through electrodes are proposed to improve the power density. Carbon paper was used for the porous electrodes and the membrane-less MFCs containing a microscale (5.4 μL) anode chamber were microfabricated in a planar monolithic cell for integration with microfluidic devices. The diffusion region between the electrolytes was numerically analyzed and fuel cell performance experiments were conducted with a wastewater inoculum-based mixed culture biofilm. The effects of the electrolyte flow rate (1-30 μL min −1 ) and electrode width (0.5-2.0 mm) on the fuel cell performance were investigated. The power density was maximized at 692 ± 34 W m −3 under optimal conditions including a 10 μL min −1 flow rate and 1.5 mm electrode width, resulting in suitable biofilm formation and low internal resistance. This study provides valuable information for the commercialization of microfluidic MFCs as a power source for portable medical and electronic instruments.