Diesel engines have to face the prospect of running on heavy and/or low cetane fuels in the future because of the expected changes in base stock and demand. The effect of physical properties and composition of fuels on the ignition delay and cetane rating is examined. The experiments were conducted on fuels having a very wide range of physical properties and C.N., in a CFR engine. The ignition delay is measured under the standard ASTM D-613 procedure and under varying needle opening pressures, and coolant temperatures. The ignition delay of some fuels is found to be dependent on the physical properties and composition of the fuels in addition to the cetane number. The cetane rating according to ASTM-D613 procedure is found to take place under hot engine conditions with a single stage ignition process. At lower compression ratios, a two stage ignition was observed. A correlation between cetane number and fuel composition was derived and found to be in agreement with the experimental results.