The NO, reduction activity on gamma-alumina was significantly improved by discharging nonthermal plasma in a simulated oxidizing exhaust gas under conditions of a high space velocity. On the other hand, those on a Pt-loaded catalyst and Cu-ZSM-5 could not be improved. The discharging nonthermal plasma converted NO to NO2 and hydrocarbons to partial-oxidized hydrocarbons, such as aldehydes, under the oxidizing conditions. From the relationship between the NOx reduction activities and the properties of several alumina catalysts, it was found that the NOx reduction activity was correlated with the number of acid sites on the alumina based on the NH3 adsorption measurement. Therefore, it is concluded that the NOx reduction occurred at the acid sites of the alumina, and the discharging nonthermal plasma improves the NOx reduction activity by NO2 formation and partial-oxidation of hydrocarbons. These results suggest that a catalytic reaction assisted by the discharging nonthermal plasma could be a promising technology for NOx reduction in lean-burn and diesel exhaust gases even under the conditions of high space velocity. (c) 2006 Society of Chemical Industry.