Fuel injection and combustion was studied with direct photography in a single cylinder DI diesel engine. Optical access was accomplished by using an endoscope-based measurement system. In the optical measurements the influence of several parameters were studied: start of injection, inlet air temperature and pressure, injected fuel amount (constant air mass), load level (varying air and fuel mass) and nozzle hole diameter. Liquid fuel spray penetration, flame lift-off and flame length were measured. The maximum spray penetration was 23 - 25 mm. As diffusion combustion started, the spray length decreased to about 15 mm. The flame lift-off was located 4 - 6 mm behind the liquid fuel spray tip. Using the two-color method the spatial temperature distribution in flames was calculated.