The graphite powder was added to a microbial fuel cell (MFC) that uses soil to decrease the internal resistance and increase the electric power and electric charge (ampere hour: Ah). The effect of adding the graphite powder to soil MFC is evaluated by experiment and simulation using mathematical models. In this experiment, the total weight of the reddish granular soil and the graphite powder was 1000 g, and ratios of the graphite were set to five values between 0% and 20% by weight. The grain size of the soil was between 3 and 8 mm. The diameter of electrode was 9 cm. The initial chemical oxygen demand (COD) of synthetic wastewater was 1400 mg/L. As a result, the amount of Ah was maximized when 10% of the graphite was mixed. When the amount of the graphite increased, the internal resistance decreased; however, consumption of COD decreased and the microbial reaction decreased. This might be because the graphite covers the surface of the soil, and it disturbs the adsorption of the soil and decreases the biofilms. Therefore, a suitable proportion of the graphite exists, and it was approximately 10% in this experiment.