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Performance improvements were studied for the diesel particulate and NOx reduction system (DPNR), a system that simultaneously reduces NOx and Particulate Matter (PM) from diesel engine exhaust gas. The experimental system (hereinafter called the “dual DPNR”) consists of two DPNR catalysts arranged in parallel, each provided with an exhaust throttle valve downstream to control the exhaust gas flow to the catalyst, plus a fuel injector that precisely controls the air-fuel ratio and the catalyst bed temperature. The fuel injector is used to supply a rich mixture to the DPNR catalyst, and the flow of exhaust gas is switched between the two catalysts by operating the exhaust throttle valves alternately. Tests were conducted with the engine running at steady state. The results indicated that the NOx reduction performance dramatically improved and the loss of fuel economy from the NOx reduction reduced.