Spray and combustion of gasoline and diesel were visualized under different ambient conditions in terms of pressure, temperature and density in a constant volume chamber. Three different ambient conditions were selected to simulate the three combustion regimes of homogeneous charge compression ignition, premixed charge compression ignition and conventional combustion. Ambient density was varied from 3.74 to 23.39 kg/m3. Ambient temperature at the spray injection were controlled to the range from 474 to 925 K. Intake oxygen concentration was also modulated from 15 % to 21 % in order to investigate the effects of intake oxygen concentrations on combustion characteristics. The injection pressure of gasoline and diesel were modulated from 50 to 150 MPa to analyze the effect of injection pressure on the spray development and combustion characteristics.Liquid penetration length and vapor penetration length were measured based on the methods of Mie-scattering and Schileren, respectively. Direct combustion visualization was also carried out to figure out the lift-off length and flame structure. Gasoline showed shorter liquid penetration length compared with diesel regardless of ambient conditions. However, vapor penetration length of gasoline and diesel was similar. Spray structure in terms of penetration and spray angle was significantly different according to the ambient conditions for both fuels. Gasoline showed increased lift-off length and increased ignition delay compared with diesel under all combustion regimes. Combustion luminosity of gasoline was somewhat lower than that of diesel due to the more homogeneous mixture from enhanced fuel-air mixing effect.