Direct and Schlieren Photographing of the combustion process in an actual D.I. diesel engine have been investigated, and by comparing both direct and schlieren methods with cylinder pressure measurement, the characteristics of mixing, ignition and burning process were analyzed. The different combustion characteristics induced by various in-cylinder air motions were also observed. The injected fuel spray jet pattern, which impinged on the combustion chamber wall, seems to affect the mixing process due to combination with in-cylinder air motion. The development of fuel jet spray was observed and compared with some theoretical treatments. The contribution of turbulence on combustion both in pre-mixed and diffusion type burning period was recognized and the effect of swirl especially on diffusion burning period was also confirmed.