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.