The behavior of V=O band over V2O5 crystallite during NH3 adsorption and SCR reaction was characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and the results are correlated with the reactivity in NH3 oxidation and SCR reaction. It is found that the decrease of V=O band intensity is due either to the reduction of V2O5 surface and/or to the adsorption of ammonia. The 70% intensity of original V=O band is preserved up to 573 K under the conditions of SCR reaction. The vanadium oxidation state is about +4.4. When the temperature reached 673 K, almost all the V=O band was recovered. From these results, it can be suggested that the decrease of the apparent SCR activity due to the increase of NO amount through NH3 oxidation above 673 K he attributed to the increase of two neighboring V=O sites, which favor the NO formation in ammonia oxidation.