The enhancement in the selective catalytic reduction of nitrogen oxides (NOx) by means of the ozone injection into the exhaust gas was investigated. Nitric oxide (NO) in the exhaust gas was first oxidized to nitrogen dioxide (NO2) by ozone, and then the exhaust gas containing the mixture of NO and NO2 was directed to the catalytic reactor where both NO and NO2 were reduced to nitrogen. The ozone injection method was very efficient for the oxidation of NO to NO2 in a wide range of temperatures, and the increase in the content of NO2 by the ozone injection remarkably improved the performance of the catalytic reactor.