Nowadays, more researches focus on protecting the environment. Present investigation concern with the effectiveness of oxygen enriched hydrogen-HHO gas addition on performance, emission and combustion characteristics of a direct injection diesel engine. Here the oxygen enriched hydrogen-HHO gas was produced by the process of water electrolysis. When potential difference is applied across the anode and cathode electrodes of the electrolyzer, water is transmuted into oxygen enriched hydrogen-HHO gas. The produced gas was aspirated into the cylinder along with intake air at the flow rates of 1 lpm and 3.3 lpm. The results show that when oxygen enriched hydrogen-HHO gas was inducted, the brake thermal efficiency of the engine increased by 11.06%, carbon monoxide decreased by 15.38%, unburned hydrocarbon decreased by 18.18%, carbon dioxide increased by 6.06%, however, the NOx emission increased by 11.19%.