A computational study aiming to investigate the effect of combustion chamber geometry on combustion process and emission has been carried out in a direct injection diesel engine. The combustion process and emission of three different combustion chamber geometries were considered, and combustion process behaviors such as variation of mean pressure, velocity, heat release rate, emission production and flame movement were revealed. The results also proved that the chamber shape has significant effects on the combustion and emission behaviors. They also showed that reentrant combustion chamber released less Soot and NOx emissions because of intense swirl, tumble and low temperature combustion. Also it was known that depth of chamber is an effective parameter on NOx and soot formation. The results of this model for cylindrical combustion chamber geometry were compared with the corresponding experimental data and proved to be good agreement. Generally it was found that the shape of Omega is the best selection for piston head, but it strongly needs strongly to match chamber geometry with spray characteristics.