Power generation characteristics of solid oxide fuel cell (SOFC) with internal steam reforming of hydrocarbons were investigated. Steam reforming reaction over a Ni-YSZ cermet catalyst attained almost the equilibrium conversion and selectivity in the fixed bed reactor at 1000 degreesC. The conversion of internal reforming of hydrocarbons was incomplete because of the limited contact time with a thick layer of the Ni cermet electrode. Therefore, the fuel cell supplied with pre-reforming gas to the anode always gave rise to a lower terminal voltage because of the insufficient conversion of fuel compared with that supplied with post-reforming gas at a given current density. Methane internal reforming proceeded without deterioration with time, whereas the power generation with ethane and ethylene suffered from carbon deposition even at high steam-to-carbon ratio. Carbon deposition region and equilibrium partial pressure of oxygen in the C-H-O diagram were estimated from the thermodynamic data. The effect of the gas composition in the power generation characteristics, especially, difference in reactivity between H-2 and CO2 was investigated. The H-2-H2O and CO-CO2 fuel systems led to almost the same open circuit voltage at the same H-2/H2O and CO/CO2 ratios at 1000 degreesC. as expected from the thermodynamic equilibrium. The output voltage in a discharge condition was always higher for H-2- H2O than for CO-CO2 at every current density.