An　experiment　was　carried　out　on　a　four-cylinder　spark-ignited　(SI)　gasoline　engine　to　investigate　the　effect　of　hydrogen　addition　on　the　engine　performance　at　low　idle　speed　conditions.　The　engine　was　run　at　three　idle　speeds　of　800　r/min,　700　r/min　and　600　r/min.　At　the　specified　idle　speed　and　stoichiometric　conditions,　the　hydrogen　volumetric　fraction　in　the　total　intake　was　gradually　increased　from　0　to　about　6%.　The　experimental　results　showed　that　the　engine　fuel　energy　flow　rate　was　reduced　from　30.8　MJ/h　at　the　idle　speed　of　800　r/min　and　a　hydrogen　volumetric　fraction　in　the　intake　of　0　to　18.6　MJ/h　at　the　idle　speed　of　600　r/min　and　a　hydrogen　volumetric　fraction　in　the　intake　of　6.0%.　For　different　idle　speeds,　the　engine　combustion　duration　was　shortened,　cyclic　variation　was　eased　and　HC,　CO　and　NOx　emissions　were　decreased　with　the　increase　of　hydrogen　volumetric　fraction　in　the　intake.　All　in　all,　the　addition　of　hydrogen　was　effective　for　enhancing　the　engine　performance　at　low　idle　speed　conditions.