The　idle　performance　of　a　hydrogen-blended　ethanol　engine　was　studied　in　a　modified　fourcylinder　spark　ignition　engine　equipped　with　electronically　controlled　hydrogen　injection　system.　Under　idle　condition,　hydrogen　injection　duration　was　gradually　increased　to　investigate　the　effect　of　hydrogen　fraction　on　the　idle　performance　of　the　ethanol　engine.　For　a　specified　hydrogen　blending　ratio,　the　ethanol　injection　duration　is　reduced　so　that　the　global　excess　air　ratio　of　air-fuel　mixtures　in　the　cylinder　can　be　kept　at　1.0.　The　test　results　show　that　the　indicated　thermal　efficiency　is　improved,　while　combustion　duration　and　cyclic　variations　are　decreased　with　the　increase　of　hydrogen　addition.　HC　emissions　decrease　whereas　NOx　emissions　increase　when　hydrogen　is　added.　CO　emission　shows　a　decrease　and　then　increase　with　the　increase　of　hydrogen　addition.　Hydrogen　addition　is　effective　on　reducing　acetaldehyde　emission　from　the　ethanol　engine.　When　hydrogen　energy　fraction　in　the　total　fuel　increases　from　0%　to　13.84%,　acetaldehyde　decreases　by　37.4%.