This paper presents simulation results of HHO gas (a mixture of H2 and O2 in a ratio 2:1 by volume - produced by the water electrolysis) addition into intake manifold of a motorcycle engine with the support of AVL BOOST software. The simulation results indicate the effects of HHO gas addition on combustion characteristics, engine power and emissions. Ignition delay and combustion duration are shortened, cylinder pressure rise rate and engine power are increased. NOx emission are reduced, however CO and HC emissions are increased with the grow of HHO content. If engine power is kept constant at 1.97 kW (at 3000 rpm engine speed and lambda equal to 0.95) fuel consumption will be saved about 12.08 percent when adding 4 liters/min HHO gas. The NOx emission concentration surges 2.4 times, while CO and HC emissions decrease 97.97 percent and 46.98 percent in compare with using gasoline.