Abstract: In order to reduce emissions of diesel engines, researchers have paid attention to the research on hydrogen as fuel of diesel engines in recent years. Investigation into the effect of the addition of hydrogen at different proportion (the highest proportion of the hydrogen is 7%) into 2004 Mack MD11 diesel engine on the emissions of CO, CO2 and HC were carried out. Results showed that corresponding to pure diesel operation, the addition of hydrogen into the engine helped to reduce the emissions of CO, CO2 and HC in various load. Moreover, as the increase of amount of hydrogen (or the more H2 added), law of the emission change for them was different with the change of load. Furthermore, the emissions of CO, CO2 and HC had a more significant reduction at low load. For example, for operation at 10% load, CO emission reduced more than 50%, CO2 emissions reduced more than 60%, and HC emission reduced more than 40%.