Decreasing the emission pollution linked with petrol combustion is gaining a great interest world-wide. Lately, HHO has been inserted to internal combustion engines as a novel clean source of energy. The effect of different HHO addition into intake manifold on the gasoline engine performance and emissions had evaluated experimentally and compared in this research. The purpose of this work is improving the performance and emissions of gasoline engine. The experimenters were performed on an air-cooled single cylinder gasoline engine with and without HHO. With the same engine loads (25%, 50%, 60%, 75% and 80%) and constant engine speed (2000 rpm) at various HHO addition into intake manifold (VR=0.037, 0.045, and 0.052) and without HHO, the engine performance parameters (BSFC and BTE) and exhaust emissions (CO, CO2, HC, and NOX) were measured. The results pointed out that: BSFC decreased, BTE incremented, CO minimized, CO2 slight increased, HC reduced and NOX diminished with increasing HHO addition at overall operation conditions when compared to gasoline fuel (without HHO).