In this experimental study, the effects of different fuel cetane numbers (CNs) and fuel injection pressures on a Diesel engine emission and on the performance were investigated. For this purpose, the fuels with 46, 51, 54.5 and 61.5 CN were tested in a four cycle, four cylinder DI Diesel engine. Measurements were conducted for each of the injection pressures 100, 150, 200 and 250 bar. Keeping 150 bar injection pressure and the other parameters constant, the changes of engine performance for the different CNs were also tested at full load condition. The results showed that NOx, SO2 and CO emissions are reduced about 15% and 5%, respectively, when the fuel CN is increased for the standard injection pressure, but the smoke value is increased dramatically when the injection pressure is reduced to 100 bar. In contrast with the lower pressure, NO, is increased and smoke is decreased when the injection pressure is increased to 250 bar. Increases in engine torque by 5% and power output by 4% were observed at the maximum torque speed of 2500 min(-1) when the CN is increased from 46 to 54.5. However, when increasing CN above 54.5, no significant increases in engine performance were observed.