In　view　of　the　high　cyclic　variation　and　partial　combustion　of　gasoline-fueled　spark　ignition　(SI)　engines　under　lean　conditions,　this　paper　investigated　the　effect　of　hydrogen　addition　to　a　gasoline-fueled　SI　engine　on　improving　its　lean　combustion　and　emissions　performance.　The　experiment　was　performed　on　a　modified　four-cylinder　SI　engine　equipped　with　an　electronically　controlled　hydrogen　injection　system.　The　engine　was　operated　at　1400　r/min,　and　a　manifold　absolute　pressure　(MAP)　of　61.5　kPa.　Two　hydrogen　volume　fractions　in　the　intake　of　1%　and　3%　were　selected　to　study　the　effect　of　hydrogen　addition　on　performance　of　a　lean　burn　gasoline　engine.　The　test　results　showed　that,　at　lean　conditions,　the　engine　brake　thermal　efficiency　was　improved,　and　flame　development　and　propagation　periods　are　shortened　with　the　increase　of　hydrogen　addition.　Moreover,　the　correlated　excess　air　ratio　for　the　lean　operating　limit　increased　from　1.45　of　the　original　engine　to　1.55　and　1.96　of　the　1%　and　3%　hydrogen-enriched　engines,　respectively.　HC　and　CO　emissions　were　decreased,　whereas　NOx　were　increased　after　hydrogen　enrichment.