The effects of diesel fuel properties and engine operating conditions on ignition delay have been studied using a single cylinder, direct injection, 4-cycle diesel engine.It was found that in the warmed up engine ignition delay increases non-linearly with decrease in the cetane number of the fuel, increasing rapidly at cetane numbers below 35. Variations in the volatility and ignition quality of the front-end of the diesel fuel were found to have no effect on ignition delay. Fuels containing cetane improver additive were found to give similar ignition delay at various torques relative to natural diesel fuels of the same cetane number.At constant RPM, increase in the engine power by increasing the fuel injection rate resulted in linear decrease in the ignition delay. Ignition delay was also shortened by increasing the intake-air pressure, as well as the oil, coolant and intake-air temperatures. Therefore, under fully warmed-up conditions, the engine was found to perform well at high torque even when using a low cetane fuel. However, under low torque and partially warmed-up conditions, the engine misfired frequently and resulted in sluggish performance.Since the cetane number (ASTM D613) is determined under one particular set of operating conditions, the validity of using it to predict performance at other operating conditions will be discussed.