Computational Fluid Dynamics (CFD), as an effective analytical tool, has been applied at China North Engine Research Institute (CNERI) for combustion chamber design and combustion system optimization on a V8 heavy -duty diesel engine in order to meet increasingly stringent emission targets. The design of combustion system involves great number of parameteric optimizations such as the number of nozzle holes, the spray angle, the swirl ratio and the piston bowl shape. 3-D CFD was a convenient and cheap tool to explore the effects of all these parameters to the engine performance, compared with extensive hardware testing.1-D modeling was used to set up boundary conditions at intake valve closure for 3-D CFD modeling during the closed-cycle. AVL FIRE software with a widely used combustion model, ECFM-3Z model, was used for 3-D simulation. Two sets of nozzle holes, four spray angles and three swirl levels were utilized and optimized under rated power. The most promising set of parameters were applied for the following optimization of the piston bowl among five shapes. Four key steady-state load points (C100, A100, A25 and rated power point) were chosen to evaluate how well each shape utilized the mixture preparation and combustion process. A merit function was selected to allow a complete assessment of emissions and power performance so as to simultaneously minimize NOx and soot emissions and maximize the indicated work. The results show that the most promising bowl shape has almost 6% higher overall merit values than the baseline.