Abstract: An experiment of hydrogen addition to a gasoline-fuelled SI engine was carried out at idle speed and at the speed of 2 000 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 2 000 r/min, the NO_x 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.