Effect of the ambient O2 concentration and pressure on the spray combustion characteristics of diesel fuel was experimentally examined using a high-temperature, high-pressure combustion vessel. The sequential images were captured by using a high-speed color video camera and were analyzed using the two color method to quantify the temporal variation of the soot temperature and KL factor. Based on a series of systematic experiments, it is confirmed that O2 concentration is the dominant factor affecting both the ignition delay and combustion period. The volumetric fraction of O2 in ambient air has great effect on flame temperature and NOx emission, however ambient pressure has little effect on both values. On the contrary both of the volumetric fraction of O2 in ambient air and the ambient pressure have large effect on soot production.