This paper investigates the impact of gas products around the anode on cell performance via an in situ observation. In a direct carbon fuel cell used this study, the anode is inserted into the carbon/carbonate slurry. The current-voltage (I-V) curves are measured before and after a long discharge in the constant current discharge mode. An in situ observation shows that the anode is almost completely covered by gas bubbles when the voltage becomes nearly 0 V in the constant current discharge at 40 mA/cm(2); this indicates that gas products such as CO2 prevent the carbon particles and ions from reaching the anode. Meanwhile, the long discharge at 20 mA/cm(2) is achieved for 30 min, even though the anode is covered by the CO2 bubbles at 15 min. The curves at 1 min after the termination of the long discharge at 20 mA/cm(2) are lower than those prior to the long discharge. The overpotential significantly increases at higher current densities, where mass transport becomes the limiting process. The cell performance is significantly influenced by the gas products around the anode. (C) 2016 Elsevier B.V. All rights reserved.