The diesel engine in the past few years has improved its market sharing because the new engine technology has reduced the emissions and the diesel engine has relatively cheaper power cost. Nevertheless, in order to meet more restricted emission standards, the NOx, CO, HC, and particulate emissions must be further reduced. Thus, this study will explore the possible fuel additive technology to further reduce the emissions from the diesel engine.In this study the fuel additives EHN, DTBP, MTBE, DMC, Diglyme, Monoglyme, and Ethanol are added into the diesel fuel with two different dosages. These additives are classified into four categories. It is well believed that the fuel cetane number improver such as DTBP (di-t-butyl peroxide) or EHN (2-ethylhexyl nitrate) can increase the fuel cetane number and thus reduce the emission level. As well as the cetane number booster, the fuel oxygenate such as MTBE or DMC which changes the oxygen composition in the fuel-air mixture can also reduce the emissions. The deposit cleaner fuel additives can substantially reduce the particulate emission due to deposit cleanup effect. The diesel engine combustion improver can accelerate the fuel evaporation and the fuel-air mixing rate so that the emission level can be reduced.Results show that the fuel additives can have substantial effects on the engine's fuel spray penetration, fuel-air mixing processes, ignition delay, chemical reaction rates, and total heat release. Certain additives have positive effects on the reduction of exhaust emissions; however, it is not necessary that the effects are all positive and significant on every emissions (Temperature, NOx, HC, and Smoke). It is not guaranteed that all these seven additives can reduce the emission concentrations, because there are certain testing results show the opposite trend in this study. Because it is not necessary that further increase of additive dosage will promote the reduction of the emission levels, there might be an optimized dosage for a specific fuel additive to bring down the emission levels.