The effect of various intake compositions and intake pressure on combustion & emission characteristics has been investigated in a single cylinder direct injection diesel engine. The variation of intake composition is simulated using argon, nitrogen and carbon dioxide as intake air diluents, and a screw compressor is used to boost intake pressure up to 200KPa. All diluents are found to be effective in reducing NOx emissions when intake pressure is changed from 110KPa to 200Kpa. Smoke emissions are drastically increased by the addition of argon, moderately increased by the addition of nitrogen. However, the addition of carbon dioxide substantially reduces smoke emissions and NOx emissions simultaneously. At lower intake pressure, the effects of diluting intake air with argon, nitrogen and carbon dioxide on ignition delay are proportional to their specific heats respectively, whereas the addition of argon has almost no effect on ignition delay when intake pressure is higher than 150KPa. Further analysis reveals that the oxygen concentration of intake charge plays the most important role in reducing Nox emissions, and the increased premixed combustion due to longer ignition delay is the main reason of reduction in smoke emissions.