An　experiment　of　hydrogen　addition　to　a　gasoline-fuelled　SI　engine　was　carried　out　at　idle　speed　and　at　the　speed　of　2000　r/min　respectively.　By　comparing　the　original　engine　and　the　engine　with　hydrogen　addition,　it　was　found　that　hydrogen　addition　can　improve　combustion,　increase　power　output　and　reduce　the　emissions　at　idle　speed.　In　addition,　with　the　improvement　of　combustion,　brake　specific　fuel　consumption　was　reduced.　While　at　the　speed　of　2000　r/min,　the　NOx　emissions　were　higher　than　that　of　the　original　engine.　In　a　word,　the　addition　of　hydrogen　to　a　gasoline-fuelled　SI　engine　is　a　good　method　to　improve　engine　performance.