Lower alcohols have very strong inhibiting effects on low temperature oxidation in HCCI combustion of normal heptane and the inhibiting effect of ethanol is slightly stronger than that of methanol while there is only weak inhibiting effect with other low ignitability fuels. This inhibiting effect of ethanol is due to significant reductions in OH radical and peroxides including HO_2, and H_2O_2 before the onsets of low temperature oxidation. The inhibiting effect of ethanol on the low temperature oxidation also suppresses the temperature rise after the low temperature oxidation, resulting in retarded high temperature oxidation. The heat release with low temperature oxidation decreases with increasing ethanol energy fraction and disappears at 30% ethanol. The onset of high temperature oxidation can be controlled by the ethanol fraction as the delay period increases linearly with the increase in ethanol. As the mol fraction of OH radicals at the onset and end of the low temperature oxidation are around 10^<-7> almost irrespective of temperature, the condition where the OH radical exceeds 10^<-7> is necessary for low temperature oxidation to progress. With ethanol addition, OH radicals are consumed in significant amount with reactions including C_2H_5OH+OH=sC_2H_4OH+H_2O and C_2H_5OH+OH=C_2H_5O+H_2O.