The chemical kinetic mechanism determines the ignition timing in homogeneous charge compression ignition (HCCI) engines. The correlation between the ignition delay in shock tubes and HCCI engines under different operating conditions was studied with the reduced mechanism of a primary reference fuel (PRF) composing of n-heptane and iso-octane. According to the similarity analysis of the sensitivity coefficient, the operating conditions which affect the similarity factor are recognized. The results indicate that, under the conditions of high pressures and the negative temperature coefficient region of the ignition delay in shock tubes, the importance of each reaction on the ignition delay in shock tubes is similar to that in HCCI engines. Furthermore, it is concluded that the effect of the ambient temperature on the ignition delay is more obvious than that of the equivalent ratio and the ambient pressure. Overall, under the conditions of large similarity factor, the performance of the chemical mechanism in shock tubes can be a good guide for the application of the mechanism in HCCI engines.