In this study, NH₃ oxidation reaction over a hydrothermally aged Cu–CHA-type SCR catalyst was studied and a model for prediction of the rate constant of the NH₃ oxidation reaction over hydrothermally aged catalyst was constructed. The results show different trends of temperature dependence on the NH₃ oxidation reaction rate above 300 ℃ for catalysts with different aging time. The structural analysis of the aged catalyst revealed that the Cu²⁺ and H⁺ active sites primarily were affected by aging and, thus, the NH₃ oxidation reaction rate at relatively lower temperatures can be determined from the number of the active sites. In contrast, the amount of CuO and the number of Cu²⁺ and H⁺ active sites can be used to determine the NH₃ oxidation reaction rate at higher temperatures.