A numerical simulation tool to predict the performance of a tubular SOFC is developed. For the convenience of the infrastructure, it has become more important to consider feeding hydrocarbon fuels like methane, which would be widely used, rather than hydrogen. Although it is well known that the performance of an SOFC drops largely when methane is fed compared with hydrogen, the reason for this is not yet well explained and thus prevents efficiently constructing an optimized SOFC system. Therefore, the present investigation is carried out to clarify how an SOFC performs when different fuels are fed. The calculation based on one-dimensional computation is carried out by introducing the parameters of actual electrode microstructures, obtained from the images taken by the focused ion beam scanning electron microscope (FIB-SEM). Values are adapted in calculating the ohmic, activation and concentration overpotentials. Proper experiments were also carried out to verify the validity of the numerical simulation. Although slight errors are found in the calculation results for fuels with high steam concentration, performances of hydrogen and methane fed cells were well predicted. Temperature distribution within the cell is also clearly shown. Through the present investigation, the performance drop, when the fuel is changed from hydrogen to methane, is found mainly caused by the temperature distribution of the cell that gives a concrete guide to construct an SOFC system.