Abstract: To lower the noise of auxiliary power system(APS) in the HEV,which is a small-volume gasoline engine in this paper,an exhaust muffler was used.The muffler was calculated and designed on the basis of a one-dimension model.The noise experiments have been carried out on a test bench.From the noise experimental results,it could be concluded that the muffler had especially much more effects on both high and medium frequencies than on low frequencies.And the muffler lowered engine noise about 5 dB during the whole engine speed without reducing the engine power much.