A predicted ignitability index for diesel combustion (the predicted diesel ignitability index) has been established with multiple regression analysis of parameters related to the bond structures in hydrocarbons as explanatory variables and the cetane numbers as a response variable. There were 116 hydrocarbons with known cetane numbers and molecular structures used for the calculations. The numbers of carbon atoms for the seven categories—C_M (carbon in a main-chain), C_SL (carbon in a side-chain longer than five atoms), C_1A (carbon in a single-benzene ring), C_2A (carbon in a double-benzene ring), C_NA (carbon in a naphtheno-benzene ring), C_1N (carbon in a single-saturated six-membered ring), and C_2N (carbon in a double-saturated six-membered ring)—were included. The predicted diesel ignitability index was expressed with these seven parameters in the following equation PDIindex=3.95CM+0.99CSL–3.99C1A–3.31C2A–2.56CNA–2.15C1N–0.96C2N+25.7 The equation of the predicted diesel ignitability index comprises one-dimensional mono-nominal formulas for each molecular structure variable, suggesting the quantitative influence of the variable on the ignitability. There is good correlation between the predicted diesel ignitability index and the cetane number, showing the coefficient of determination with an R² of 0.82. The simulation was validated with the ignition delays of 31 blends of hydrocarbons and gas to liquid in a diesel engine. The predicted diesel ignitability index showed better correlations with the measured ignition delays than the cetane number at all three intake oxygen concentrations examined here. Especially, at a low intake oxygen concentration, there was significant scattering between the cetane number and the ignition delay for low cetane number fuels, where the predicted diesel ignitability index showed much smaller scattering.