Cu-SSZ-13 is widely used as a material for Cu-SCR catalyst. The Cu-SCR catalyst shows high NOx performance and has high thermal durability but it deteriorates in NOx performance when it suffers Sulfur poisoning. Authors investigated the detailed mechanism how the catalyst is poisoned by Sulfur, and also studied the optimum de-Sulfation conditions. As to the Sulfur adsorption site in the Cu-zeolite, we performed DFT calculation to know the site candidate and we performed precise characterization. As characterization techniques of Sulfur poisoning of the catalyst, we mainly used EUPS (Extreme Ultraviolet Photoelectron Spectroscopy) and DRIFTS. By those techniques, we found out that Sulfur adsorbs on ion-exchanged Cu site and Al site in the Zeolite structure. Especially the Cu site is an active site of the catalyst and thus the Cu-SCR catalyst deteriorated by the Sulfur poisoning. Regarding to the de-Sulfation conditions, we changed the temperature, atmosphere and SV (space velocity) and Ea (activation energy) was compared on each conditions. At preferable lower temperature, higher de-Sulfation was seen with co-existence of reducing agent, here NH3. Details of information and data will be disclosed and discussed on the manuscript.