Pressurized SOFCs operate at high temperatures and generate electricity by using various fuel species. SOFCs can be applied for large-scale power generation combined with gas and steam turbine(s) under high pressure. In this study, fuel gas composition is systematically considered by thermochemical calculations on the anode side of the SOFCs pressurized up to 30 bar in a wide temperature range up to 1000oC. In general, fuel gas consists mainly of carbon, hydrogen, and oxygen, so that the C-H-O equilibrium diagrams have been calculated for different cases. It is revealed that carbon deposition region contracts in the hydrogenrich area and expands in the oxygen-rich area with increasing the total pressure. The molar fraction of each gas component also depends also on the total pressure.