The relationship between the oxidation state of Cu supported on an alumina catalyst (Cu/Al2O3) and the activity for combustion of NH3 was investigated. Combustion of NH3 on the catalyst treated in hydrogen at 800 degrees C occurred at lower temperature than on the catalyst treated in air. It was also much better than Pt and Ph catalysts for the conversion of NH3 to N-2. Characteristics of the catalyst were investigated by XRD, XPS, and the N2O pulse injection method to understand the reason of its high catalytic activity. The reason of the high activity of the catalyst treated in hydrogen at the high temperature was attributed to the lower oxidation state of Cu in the catalyst.