Abstract: Cobalt oxide/graphite (Co₃O₄/G) composite was prepared and applied for the first time in marine sediment microbial fuel cells (MSMFCs) to improve the performances of anode and cell. The amount of microbes on the surface of Co₃O₄/G composite modified anode is 6. 1 times of that of the blank group. The redox electrochemical activity and capacitance of composite modified anode are 16. 2 times and 31. 0 times as high as those of the blank group, respectively; its exchange current density is 1. 366 × 10^-3m A cm^-2 and its electron transfer kinetic activity is 215. 6 times of that of the blank group; its anti-polarization ability is the strongest; the charge transfer resistance is reduced to 2/5 of that of the blank group, and the electric double-layer capacitance and biofilm capacitance are both improved; the power density of the cell is 735. 1 m W/m², which is 4. 6 times of that of the blank group. The mechanism analysis shows that the capacitance performance and the electron transfer rate of the composite modified anode are attributed to the synergistic effect of Co₃O₄ and graphite.