Efficient catalytic reduction of nitrogen oxides (NO_x) at low temperature can be achieved by injecting ozone into the exhaust gas. For the present study, a dielectric barrier discharge device using a high-voltage ac was employed to produce ozone, which was continuously fed to the exhaust gas, resulting in the oxidation of NO to NO₂. The experiments were primarily concerned with the influence that the NO₂content in NO_x(NO+NO₂) has on the catalytic reduction of NO_x. The changes in the NO_x content that occurred as a result of the ozone injection were found to largely affect the catalytic reduction of NO_x, especially at low temperature. Using a simplified kinetic model and several sets of experimental data, the rate constants of the main reactions contributing to the NO_x reduction were determined. To verify the validity of the rate constants, the results calculated by the kinetic model were compared with the experimental data obtained by changing the reaction temperature, the feed gas flow rate, and the initial NO_x concentration. The agreements between the experimental and calculated results were quite good for the cases involving either the presence or absence of ozone injection.