The performance behavior of solid oxide fuel cell using H₂ and CO as fuels was investigated. The power densities and impedance results showed a little variation as the ratio of H₂ and CO changed. However, when the pure CO was used as a fuel, area specific resistance (ASR), especially low frequency region, was increased. This might be due to carbon deposition on anode. The maximum power density was 60% lower using CO than using H₂. Carbon deposition reduced after constant current was applied. The SOFC performance was recovered from the carbon deposition after applying constant current during 100h.