The investigation on the durability of solid oxide fuel cell (SOFC) stacks in real operation environments has revealed that the cathode is contaminated with sulfur during SOFC operation, although the sulfur dioxide concentration is very low in air (in ppb level). In this work, the effect of low concentration SO₂ (0.01ppm) in air on the degradation of a La_0.6Sr_0.4Co_0.2Fe_0.8O₃ (LSCF6428) cathode was investigated with different operation temperature of 650 ^oC, 700 ^oC, and 800 ^oC for 300 h. The performance degradation of LSCF6428 cathode is most serious at 700 ^oC due to both of ohmic and polarization resistance increase. The sulfur distribution and the SrSO₄ formation concentrate near TPB at T ≤ 700 ^oC, but are near cathode surface at 800 ^oC. These different morphology changes of cathode near TPB are the main reason for the different performance degradation of the LSCF6428 cathode at different temperature.