The effect of methane as fuel on the performance of a micro tubular type solid oxide fuel cell (SOFC) was investigated using electrochemical analysis. The species and its composition of various exhausted gases from a micro tubular SOFC was identified by a mass spectroscopy analysis. Three electrochemical reactions were processed simultaneously during the operation. Hydrogen gas as fuel diluted with argon by various ratios was included as a standard baseline performance. The results showed that the performance was dropped as the hydrogen was diluted. After applying diluted methane to the cell, an extra amount of carbon which is an evidence of accumulated carbon was observed at the outlet. An extra cathodic reaction could happen during this operation depending on the current and potential applied. The carbon on anode might be accumulative and troublesome for this analysis. It was found that this carbon deposition was smaller when the voltage was lower at high currents but it was increased slightly at higher potentials at low current. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3624613] All rights reserved.