Depending on the diversification of liquid fuel sources, changes in diesel fuel composition are expected. These changes may increase the gap between cetane index and cetane number. Therefore, enhanced cetane index formula which has higher predictive performance is required. In this study, we focused on the chemical structures which affect the auto-ignition characteristics. Specifically, the chemical structures mean carbon number, number of side chains, the rate of naphthenic rings, and the rate of aromatic rings. Characteristics of these structures in diesel fuel can represented by the specific combination of physical properties, namely, distillation characteristic, kinetic viscosity and density. Now, therefore, enhanced cetane index formula is described by three physical parameters mentioned above.