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NOx conversion to N2 was investigated by a plasma/catalyst system in rich oxygen similar to a lean-burn engine exhaust. Catalytic activity was enhanced by the assistance of plasma, and the plasma/catalyst system showed higher NOx conversion over a wider temperature window than did catalyst alone. The plasma/catalyst (Ag/Al2O3) system showed a remarkable improvement in NOx conversion in the lower temperature range under 400 °C. The NOx conversion of plasma/catalyst (Ag/Al2O3) was 40%–50%, and its selectivity to N2 was over 96% under the conditions that the hydrocarbon (C1 base) to NOx ratio was about 3 and 10% O2 existed over the temperature range of 300–500 °C. Ag/Al2O3 showed a better performance than Al2O3 when assisted by plasma. Helium was used as a balance gas to measure N2 formation by gas chromatography, and NOx conversion was measured by a chemiluminescence NOx analyzer in N2 balance as well as He balance. The result in N2 balance gas was compared with the result in helium balance gas at the 300 °C where the largest enhancement by plasma was observed. The NOx conversion in N2 balance gas was similar to that in helium balance gas, which verified that the plasma effect obtained in helium balance gas is consistent with that of N2 balance gas, which is the largest component of the real exhaust gas.