The diesel combustion and emissions with four kinds of oxygenated agents were investigated as main fuel in high speed DI diesel engines. Simultaneous improvements in smoke, particulate matter, NO_x, THC, and thermal efficiency were obtained for all four agents, and engine noise was also reduced for the two agents with higher ignitability. The improvements in the exhaust gas emissions and the thermal efficiency almost only depended on the oxygen content in the fuels regardless of oxygenated agents-diesel fuel blending ratios and kinds of oxygenated agents. Completely smokeless operation with simultaneous improvements in NO_x, THC, noise, and thermal efficiency was realized with some oxygenated fuels including, diethylene glycol dimethyl ether (DGM). The rate of heat release diagram of the DGM shows an optimum combustion with a milder first stage combustion, more active main stage combustion, and shorter combustion period. The unburnt THC and odor intensity under starting operation with DGM were much lower than with diesel fuel only.