The present study examines the global effect of a laboratory-simulated exhaust gas recirculation (EGR) on the homogeneous charge compression ignition (HCCI) combustion of n-heptane in a transparent monocylinder diesel engine. The investigations were carried out during the engine cycle for two EGR rates, 0 and 30%, at a constant 0.3 equivalence ratio. The analysis is focused on the relation between natural combustion emissions during the cold and main combustion phases and the OH* chemiluminescence and the processes involved inside the combustion chamber of a compression engine. By observing cool and main flame emissions, it was observed that the dilution by burnt gas delays and degrades the combustion phenomenon. For both cases, the natural flame emission is not uniformly distributed, because of the nonhomogeneity of the gas temperature. By coupling kinetic analysis, provided by a zero-dimensional model, it was shown that the natural emissions of combustion are sufficiently sensitive to yield combustion process analysis.