Recently, using hydrogen or hydrogen-rich gas as a supplement fuel for spark ignition and compression ignition engines is one of the potential solutions for improving brake thermal efficiency, reducing fuel consumption and pollution emissions from internal combustion engines. This article investigates the effect of HHO gas addition on engine performance and emission characteristics.HHO gas was produced by the electrolysis process of distilled water and stored in a high pressure tank before injected into the intake manifold. The experimental study was carried out on a 97 cc SI engine equipped with two injection systems (HHO gas and addition air) on the intake manifold. The tests were divided into two cases: hybrid HHO/gasoline and HHO/gasoline with addition air from second injection. The experiments showed that, of both cases, compared to original engine, the engine performance was improved and the gasoline fuel consumption was declined after enrichment of HHO gas and of HHO gas/addition air mixture,. The NOx emission was increased; however,, HC emission was reduced. The CO and CO₂ emissions displayed different trends between the two cases. When only HHO gas was injected, the CO emission surged due to rich mixture, while it was decreased after supplying the addition air in the second case. The CO₂ emission trend was in opposite direction of CO. The study demonstrated that the effect of HHO gas addition is most apparent at light loads and lean conditions.