Abstract: In order to improve engine performance, chemical kinetic mechanisms of diesel and ethanol/diesel blends were studied. The influences of important species, free radicals and the key reactions on homogeneous charge compression ignition (HCCI) engine fueled with ethanol/diesel blends and conventional diesel were compared. The results showed that with the introduction of ethanol in diesel oil, the first O2 addition and the second O2 addition play more important roles in the thermal decomposition of C7H15. The influences of different intake temperature (400 K、420?K and 440 K) and different equivalence ratio (0.2、0.4 and 0.6) on HCCI combustion were also investigated. The initial temperature has important influence over the timing of ignition, and the equivalence fuel/air ratio has significant effect on in-cylinder temperature, in-cylinder pressure and the timing of ignition. With the increase of charge temperature, H-abstraction reaction and O2 addition were advanced and enhanced but then decreased. With the increase of equivalent fuel/air ratio, H-abstraction reaction and O2 addition were delayed and enhanced, and the period of reaction was shortened. The results are reasonable and can be used in the performance control on engine fueled with ethanol/diesel blends.