NO x conversion to N 2 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 NO x conversion over a wider temperature window than did catalyst alone. The plasma/catalyst (Ag/Al 2 O 3 ) system showed a remarkable improvement in NO x conversion in the lower temperature range under 400 °C. The NO x conversion of plasma/catalyst (Ag/Al 2 O 3 ) was 40%–50%, and its selectivity to N 2 was over 96% under the conditions that the hydrocarbon (C 1 base) to NO x ratio was about 3 and 10% O 2 existed over the temperature range of 300–500 °C. Ag/Al 2 O 3 showed a better performance than Al 2 O 3 when assisted by plasma. Helium was used as a balance gas to measure N 2 formation by gas chromatography, and NO x conversion was measured by a chemiluminescence NO x analyzer in N 2 balance as well as He balance. The result in N 2 balance gas was compared with the result in helium balance gas at the 300 °C where the largest enhancement by plasma was observed. The NO x conversion in N 2 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 N 2 balance gas, which is the largest component of the real exhaust gas.