High temperature fuel cells such as the solid oxide fuel cell (SOFC) are considered extremely suitable for electrical power plant application. In order to better understand the thermodynamics of fuel cell and cycle’s co-flow planar SOFC model are built in Aspen Custom Modeller for the externally reformed (ER) SOFC and internally reformed (IR) SOFC. These models are integrated in Aspen PlusTM as externally and internally reformed SOFC systems. In this article the influence of the steam carbon ratio (S/C) on the performance of a Solid Oxide Fuel Cell System is investigated. It is found that the cell voltage and the gross and net efficiencies of both SOFC are higher in case of S/C=2 compared to S/C=3.56. The SOFC simulations show that the cycle with internal reforming gives better efficiencies than the cycle with external reforming.