The de-NOx performance of sewage sludge chars activated physically using water vapor or chemically using KOH was evaluated. Characteristics of chars activated under different activation temperatures, water vapor concentrations, and KOH/char ratios were analyzed by N2 adsorption-desorption and FT-IR. Chemically activated chars were more efficient in removing NOx due to their higher surface area, oxygen functional group and NH3 adsorption than physically activated chars. The de-NOx performance of the activated chars is influenced more by chemical properties such as oxygen functional groups and NH3 adsorption sites than by physical properties like specific surface area and pore volume.