The electrochemical oxidation of carbon was studied in a direct carbon fuel cell (DCFC) using a carbon/carbonate slurry stirred by Ar bubbling. First, a single carbon pellet in contact with an anode in molten carbonate was observed during discharge. Bubbles were produced from the carbon during the discharge, indicating that the electrochemical oxidation of the carbon was advanced. The performance of the DCFC using activated carbon was then studied. By increasing the Ar bubbling flow rate (Q(bub)) from 25 to 50 mL/min, the overpotential increased at high current density where the concentration polarization appeared; whereas, the overpotential at Q(bub) = 25 mL/min was almost the same as that at Q(bub) = 50 mL/min at lower current density than 20 mA/cm(2) and coulombic efficiency increased from 78 to 93% at 20 mA/cm(2). The bubbling optimization is expected to improve DCFC performance. Coulombic and voltage efficiencies decreased with increasing the current density, while the power density remained the same. The performance of the DCFC using coal chars was also studied. Ar bubbling was effective in the DCFC using coal chars.