The impact of fuel injection pressure on the development of diesel flames has been studied in a light-duty optical engine. Planer laser-induced fluorescence imaging of fuel (fuel-PLIF) and hydroxyl radicals (OH-PLIF) as well as line-of-sight integrated chemiluminescence imaging of cool-flame and OH* were performed for three different common-rail pressures including 70, 100, and 130 MPa. The injection timing and injected fuel mass were held constant resulting in earlier end of injection for higher injection pressure. The in-cylinder pressure was also measured to understand bulk-gas combustion conditions through the analysis of apparent heat release rate. From the cool-flame images, it is found that the low-temperature reaction starts to occur in the wall-interacting jet head region where the fuel-air mixing could be enhanced due to a turbulent ring-vortex formed during jet-wall interactions. Also, the cool-flame images together with the apparent heat release rate suggest that the low-temperature reaction becomes stronger with increasing injection pressure. The influence of in-cylinder swirl flow on the OH* chemiluminescence signals was observed such that the high-temperature reaction on the down-swirl side of the jet is earlier than that on the up-swirl side of the jet regardless of the injection pressure. Moreover, the second-stage ignition on the down-swirl side of the jet is found to be stronger than the up-swirl side of the jet. This is consistent with higher fuel penetration and more intense fuel fluorescence signals observed on the down-swirl side of the jet, suggesting that relatively richer mixtures caused stronger high-temperature reactions. As the injection pressure increases, however, the spread and magnitude of the up-swirl OH* chemiluminescence signals become comparable to the down-swirl signals due to the increased injection momentum overcoming the swirl flow. On account of the unavoidable laser beam attenuation, the OH-PLIF signals on the up-swirl side could not be observed clearly. The down-swirl OH-PLIF signals, however, display interesting behaviour of the wall-interacting jet head such that the OH-PLIF region expands drastically in the turbulent ring-vortex region where the cool-flame signals were detected at earlier timings. The expansion of wall-jet head OH signals shows an increasing trend with increasing injection pressure, which is explained by a stronger ring-vortex due to the increased injection momentum.