The effect of fuel type, fuel molar concentration, and air molar concentration on the autoignition temperature (AIT) of n-heptane, methanol, ethanol, and butanol, under a wide range of conditions (phi = 0.4-2.0, P-init = 80-500 psi), is investigated using a constant volume bomb. The results indicate that the AIT generally decreases as the chain length and molecular weight each increase. The AIT decreases as the fuel molar concentration, as well as air molar concentration, increase. The MT of n-heptane remains approximately constant (280 degrees C) as the fuel/air molar concentration increase. While the MT of methanol, ethanol, and butanol decrease from 548 degrees C to 479 degrees C, from 450 degrees C to similar to 371 degrees C, and from 401 to 342 degrees C, respectively, with increasing fuel/air molar concentration. Numerical study is performed to identify the cause of AIT variation using validated comprehensive reaction mechanisms of n-heptane and methanol. The AIT of n-heptane remains almost constant under wide parameters, because of the low activation energy and multiple reaction pathways of keto-hydroperoxides formation and decomposition, which are the chain branching reactions of n-heptane low-temperature oxidation. Conversely, the AIT of methanol is susceptible to external factors such as fuel/air molar concentration may be attributed to the high energy barriers and relatively simple routes of H atom abstraction from methanol by hydroperoxyl radical and hydrogen peroxide decomposition, which are the chain branching reactions of methanol oxidation.