Interaction of fuel spray and surrounding air plays important role to mixture formation, ignition and combustion processes in diesel combustion. Effects of supercharging, swirl strength and fuel injection pressure on development, ignition and combustion process of diesel spray were investigated by using free pisto type rapid compression machine. Detailed observation of the spray was carried out by visualization including schlieren photography and direct photography. The experiments show the results as below. There is an optimum strength in swirl for favorable mixture formation. Higher supercharging pressure makes swirl more effective. Higher injection pressure increases the amount of well-prepared mixture prior to ignition, thus it results less smoke.