The　effect　of　syngas　addition　on　the　performance　of　a　1.6　L　gasoline　engine　at　lean　condition　was　investigated　in　the　paper.　The　syngas　which　produced　by　the　onboard　ethanol　catalytic　decomposition　was　mainly　composed　of　hydrogen　and　carbon　monoxide.　A　tube　array　reforming　reactor　was　mounted　on　the　engine　tailpipe　to　produce　syngas.　During　the　test,　the　engine　was　run　at　1800　rpm　and　a　manifolds　absolute　pressure　of　61.5　kPa.　The　spark　timing　for　the　maximum　brake　torque　was　adopted　for　all　tests.　The　engine　spark　timing,　injection　timing　and　duration　of　the　gasoline　were　controlled　by　a　hybrid　electronic　control　unit　communicated　with　the　engine　original　electronic　control　unit.　The　syngas　volume　fraction　in　the　total　intake　gas　was　gradually　increased　from　0%　to　1.84%.　The　gasoline　flow　rate　was　decreased　to　ensure　that　the　global　excess　air　ratio　of　the　fuel-air　mixture　in　cylinder　at　about　1.20.　The　test　results　confirmed　that　the　syngas　addition　helped　improve　the　indicated　thermal　efficiency　and　shorten　the　combustion　duration.　HC,　NOx　emissions　and　particle　total　number　per　cubic　centimeter　were　reduced　after　the　syngas　addition　at　lean　condition.　Copyright　(C)　2013,　Hydrogen　Energy　Publications,　LLC.　Published　by　Elsevier　Ltd.　All　rights　reserved.