A plasma-catalytic combined process was used as an attempt to improve the conversion efficiency of nitrogen oxides (NOx) over a wide temperature range (150~500 ℃) to cope with the exhaust gas whose temperature varies greatly. Since the catalytic NOx reduction is effective at high temperatures where the activity of the catalyst itself is high, the NOx reduction was carried out without plasma generation in the high temperature region. On the other hand, in the low temperature region, the plasma was created in the catalyst bed to make up for the decreased catalytic activity, thereby increasing the NOx conversion efficiency. Effects of the types of catalysts, the reaction temperature, the concentration of the reducing agent (n-heptane), and the energy density on NOx conversion efficiency were examined. As a result of comparative analysis of various catalysts, the catalytic NOx conversion efficiency in the high temperature region was the highest in the case of the Ag-Zn/γ-Al2O3 catalyst of more than 90%. In the low temperature region, NOx was hardly removed by the hydrocarbon selective reduction process, but when the plasma was generated in the catalyst bed, the NOx conversion sharply increased to about 90%. The NOx conversion can be maintained high at temperatures of 150~500 ℃ by the combination of plasma in accordance with the temperature change of the exhaust gas.