Abstract: A reduced n-heptane/iso-octane/ethanol/PAH mechanism was proposed and validated with various experimental data. Based on this mechanism, simulations were carried out to investigate the combustion characteristics of n-heptane spray under premixed ethanol/air and iso-octane/air atmosphere. The effects of premixed ethanol and iso-octane on ignition delay, important soot precursors and soot volume fraction of n-heptane spray under different ambient temperatures were studied. And the simulated results with and without considering the cooling effects of premixed fuel vaporization were compared. When the cooling effect of premixed fuel vaporization was not considered, the simulation results showed that the premixed ethanol could increase the ignition delay of n-heptane spray for ambient temperature below 850K, while the inhibition effect of ethanol on ignition disappeared when the ambient temperature was further increased. Compared with ethanol, the inhibition effect of premixed iso-octane on ignition was slightly weaker. However, premixed fuel contributed to faster ignition under high ambient temperature. Meanwhile, the equivalence ratio of premixed fuel has obvious effect on ignition delay of n-heptane spray. The soot volume fraction and soot precursor concentration were significantly reduced under premixed ethanol and iso-octane atmosphere at low temperature. When the cooling effect of premixed fuel vaporization was considered, the simulation results with premixed iso-octane present little difference. However, the simulations showed that premixed ethanol could delay the ignition of n-heptane spray even at 1000K. Also the soot volume fraction was significantly reduced under premixed ethanol atmosphere for ambient temperature below 900K. Therefore, compared with the low reactivity of ethanol, the high vaporization of ethanol also has an obvious effect on ignition delay of n-heptane spray.