This work studied internal loss factors (flow rate, pressure, partial pressure, voltage, current, temperature, etc.) in a unit cell and stack of solid oxide fuel cells (SOFC) with different separator materials, operation temperature and types of gas supply channels by employing computational fluid dynamics (CFD). A steel separator (AISI430) was superior to a ceramic plate (LaCrO3 ) in an aspect of thermal stress due to high thermal conductivity but inferior at average current density and fuel utilization rate. As initial temperature at the cell inlet was lowered from 950 °C to 650 °C per each pattern of gas flow (co-flow and counter-flow), useful data were acquired to analyze a performance drop. I-V curves at 650 °C and 900 °C, which involved various parameters as separator materials and directions of gas supply, compared performance characterization between low and high temperature SOFC and also implied the most effective combination.