A direct carbon fuel cell (DCFC) system directly converts the chemical energy of solid carbonaceous fuel into electrical energy. The electrochemical reaction of this system has an influence on the properties of solid fuel, such as crystal structure, element composition, and surface properties. In addition, when using raw coals as DCFC fuel, the volatile gases released from coal at a high temperature affect cell performance. The purpose of this study is to investigate the effect of fuel characteristics on the resistance of the inner DCFC system by electrochemical impedance spectroscopy (EIS) and equivalent circuits. Two kinds of solid carbon, graphite and sub-bituminous coal, were prepared to compare electrochemical characteristics by EIS measurement. The equivalent circuits were applied to the constant phase element (CPE) in a Randle circuit to explain the correlation between the fuel characteristic and electrochemical reaction resistance.