A simple and low-cost modification method was developed to improve the power generation performance of inexpensive semicoke electrode in microbial fuel cells (MFCs). After carbonization and activation with water vapor at 800–850 °C, the MFC with the activated coke (modified semicoke) anode produced a maximum power density of 74 W m−3, 17 W m−3, and 681 mW m−2 (normalized to anodic liquid volume, total reactor volume, and projected membrane surface area, respectively), which was 124 % higher than MFCs using a semicoke anode (33 W m−3, 8 W m−3, and 304 mW m−2). When they were used as biocathode materials, activated coke produced a maximum power density of 177 W m−3, 41 W m−3, and 1628 mW m−2 (normalized to cathodic liquid volume, total reactor volume, and projected membrane surface area, respectively), which was 211 % higher than that achieved by MFCs using a semicoke cathode (57 W m−3, 13 W m−3, and 524 mW m−2). A substantial increase was also noted in the conductivity, C/O mass ratio, and specific area for activated coke, which reduced the ohmic resistance, increased biomass density, and promoted electron transfer between bacteria and electrode surface. The activated coke anode also produced a higher Coulombic efficiency and chemical oxygen demand removal rate than the semicoke anode.