This study computationally investigates the combined effects of EGR and boost pressure on HCCI combustion process for n-heptane under the condition of constant amount of fuel. The computations were conducted using the single-zone model of CHEMKIN-PRO with detailed chemical-kinetics mechanisms for n-heptane from Lawrence Livermore National Laboratory. Contribution matrix on heat release of elementary reactions is employed to better understand underlying mechanisms of EGR and boost pressure on HCCI combustion process by extracting important elementary reactions. Using contribution matrix on heat release of elementary reactions, it is possible to chemically explain the combined effects of EGR and boost pressure on HCCI combustion process.