The spectral analysis of chemiluminescence of a PCCI diesel engine operating with moderately early injection timing was carried out in a single cylinder test engine. Investigations were conducted through direct visualization of the combustion phenomena with spectroscopic measurement of the intermediate species using a spectrometer coupled to ICCD camera. In order to understand the chemical kinetics of auto-ignition and combustion mechanism in PCCI engine, the spectral analysis of chemiluminescence was carried out. The following intermediate species were identified; OH*, CH*, CO-O recombination and formaldehyde (CH2O) which marked the different combustion phases. Formaldehyde was detected in the low temperature oxidation (LTO) region and isolated clearly by high spectral resolution. The OH* radicals marked the regions with premixed fuel-air mixtures and high temperature oxidation (HTO) region. CH* radicals were noted to mark well the fuel-rich region within the piston cavity after fuel impingement on the Derby hat wall. Soot formation arising from the fuel impingement on the Derby hat wall was oxidized in the late combustion phase. High temperature indicated by high intensity of OH* radicals led to faster oxidation of the soot in the late combustion phase. Copyright © 2012 by the Japan Society of Mechanical Engineers.