Ignition process chemistry was analyzed using a detailed chemical kinetic model of n-heptane generated by KUCRS (Knowledge-basing Utilities for Complex Reaction Systems), wherein pressure-dependent rate constants of the O₂ addition to alkyl radicals and hydroperoxy alkyl radicals and the thermal decomposition of ketohydroperoxides have been introduced. Then, the effect of the initial pressure and the individual effects of the initial fuel, O₂ and N₂ molar concentrations on a relationship between the initial temperature and the ignition delay were discussed. When the initial temperature increases, the branch of C₇H₁₄OOH removal into the second O₂ addition and the decomposition into C₇H₁₄cyO and OH is more sensitive to the pressure and the O₂ concentration, and thus, the LTO preparation phase is more affected by the pressure and the O₂ concentration. The LTO phase terminates mainly by the OH removal by intermediate species. When the pressure and the O₂ concentration increase, the activated second O₂ addition to C₇H₁₄OOH causes intermediate species to accumulate less efficiently, and thus, the LTO end temperature to increase. A period of the thermal ignition preparation phase is controlled by the rate of H₂O₂ (+ M) = OH + OH (+ M). When the pressure increases, the rate of this reaction increases by the dependence order of about 2, and due to the proportional increase in the whole gas concentration, the ignition delay shortens by the dependence order of about 1 in the blue-flame dominant region.