Concerning　the　high　cyclic　variations　and　emissions　of　spark　ignition　(SI)　engines　at　idle,　this　paper　experimentally　investigated　the　effect　of　hydrogen　addition　to　a　gasoline　fueled　SI　engine　on　improving　its　idle　performance.　The　experiment　was　performed　on　a　modified　1.6　L　SI　engine　equipped　with　an　electronically　controlled　hydrogen　port-injection　system.　The　injection　duration　and　timing　of　hydrogen　and　gasoline　were　governed　by　a　self-developed　electronic　control　unit.　The　hydrogen　volumetric　fraction　in　the　total　intake　gas　was　gradually　increased　from　0%　to　6.52%.　At　a　specified　hydrogen　addition　level,　the　gasoline　flow　rate　was　reduced　to　keep　the　engine　operating　under　the　stoichiometric　condition.　The　test　results　demonstrated　that　the　flame　development　and　propagation　durations　were　decreased　with　the　increase　of　hydrogen　adddition　fraction.　The　coefficient　of　cyclic　variations　in　indicated　mean　effective　pressure　was　reduced　from　9.97%　of　the　original　engine　to　4.93%　of　the　6.52%　hydrogen-enriched　engine.　NOx　emissions　were　monotonously　decreased　with　the　increase　of　hydrogen　addition.　HC　and　CO　emissions　firstly　decreased　with　the　increase　of　hydrogen　addition,　however,　when　hydrogen　volumetric　fraction　in　the　intake　was　larger　than　4.88%,　HC　and　CO　emissions　turned　to　increasing　with　the　increase　of　hydrogen　addition.