The fuel-ambient gas interaction process of the free diesel spray injected from the micro-hole nozzle (0.08 mm) into the quiescent and engine-like ambient gas condition was investigated by means of the laser-induced fluorescence - particle image velocimetry (LIF-PIV) technique in non-evaporating condition. Direct photography with high speed video camera and two color pyrometry were applied to analyze the evaporation spray and flame characteristics. Three injection pressures from 100, 200 to 300 MPa and two ambient gas densities of 11 and 15 kg/m₃ were selected as testing conditions. The entrained mass flow rate of the ambient gas through the whole spray boundary, the ratio of the total ambient gas entrainment rate to the fuel injection rate, etc., were calculated by using the ambient gas velocity data obtained by the LIF-PIV technique and used to correlate the combustion behavior. The relationship between the ambient gas entrainment and the spray evaporation and combustion characteristics was analyzed. The higher injection pressure improves the fuel/ambient gas mixing and fuel vaporization, which results in the lower soot formation in the spray flame. The higher ambient gas density tends to decrease the premixed combustion portion and increase the soot formation since the ignition delay and lift-off length for ambient gas entrainment are shortened significantly. More ambient gas mass could be entrained upstream of the soot formation position in the case of higher injection pressure and lower ambient gas density, which improves the premixed combustion phase. A general model of the effect of injection pressure and ambient gas density on the liquid phase and soot level is introduced based on the ambient gas entrainment through the spray side surface.