For intermediate temperature operation, we chose an anode-supported, planar type SOFC (Solid Oxide Fuel Cell) design considering mass production with use ferritic stainless steels as cost-effective interconnects. Anode-supported single cells with thin electrolyte layer of YSZ(Yttria-Stabilized Zirconia) were fabricated and short stacks were built and evaluated. We also developed diesel and methane autothermal reforming(ATR) reactors in order to provide fuels to SOFC stacks. Influences of the H2O/C(steam to carbon ratio), O2/C(oxygen to carbon ratio) and GHSV(Gas Hourly Space Velocity) on performances of stacks have been investigated. Performance of the stack operated with a diesel reformer was lower than with using hydrogen as a fuel due to lower Nernst voltage and carbon formation at anode side. The stack operated with a natural gas reformer showed similar performances as with using hydrogen. Effects of various reformer parameters such as H2O/C and O2/C were carefully investigated. We found O2/C is a sensitive parameter to control stack performance.