An experimental study aiming to investigate the effects of combustion chamber geometry on combustion process has been carried out in an optically accessible DI diesel engine. The combustion processes of three different chamber geometries, included the production type, were revealed and the flame movement behaviors such as the distribution of flame velocity vectors and the averaged flame velocity inside and outside the combustion chamber were measured by means of a cross-correlation method. Meanwhile, an endoscope system was used to acquire information about the distribution of flames inside and outside the chamber. BY comparing the flame movement and distribution between different chambers and nozzle protrusions, the results showed that; The chamber geometry has significant effect on the flame velocity, the flame velocities of the reentrant chamber were larger than that of the dish chamber during expansion period. The flame distribution inside and outside the chamber is considerably affected by the chamber wall geometry, and the reentrant chamber prevents the flame from spreading over the squish region.The relationship between these changes of the flame behaviors and its smoke emissions measured in the same engine under the same experimental conditions are also discussed. This relationship has indicated that for further smoke reduction, it is important to achieve large flame motion during diffusion combustion and desirable fuel/flame distribution inside and outside the chamber simultaneously.