A novel approach based on the chemical kinetics has been applied to elucidate the ignition phenomena in spray combustion. Cetane number was suggested to correlate with the ignitability of fuel-rich mixtures (not with those of stoichiometric mixtures), or with the cool-flame ignition delay time. A fuel-air adiabatic mixing model was used to estimate the local equivalence ratio of the mixture which is most relevant to the ignition of fuel spray. Also, the effect of the possible mixing of gas parcels with different equivalence ratios and thus different history of oxidation was discussed based on the simplified model calculations.