This paper shows development challenges for 6 liter heavy duty off-road diesel engines to meet the Tier4 final emission regulations with a base diesel engine compliant with Tier4 interim emission regulations. Even if an after-treatment system helps to reduce emissions, quite amount of particulate matters (PM) reduction is still necessary since a diesel particulate filter (DPF) system is supposed to be excluded in Tier4 final diesel engine.The objective of this research is to see if the base engine has a feasibility to meet Tier4 final emission regulations by a change of piston bowl geometry without DPF. Quite amount of PM can be reduced by piston bowl geometry because piston bowl geometry is a very important part that enhances air and fuel mixing process that help the combustion process. Local air to fuel ratio and mixing rate could be improved by manipulating the gas motions of in-cylinder and PM could be reducedIn this study, multidimensional CFD code KIVA-3V coupled with detailed chemical kinetics is used to perform combustion simulations. The validation between engine experiment results and simulation results was established at first. Then, with visualized in-cylinder images analysis, geometrical parameters were changed from the conceptual ULPC (Ultra Low Particulate matter Combustion) piston bowl geometry. After several iterations of analysis and geometrical parameter changes, main geometrical parameters were finally found and optimized piston bowl geometry was obtained.To prove the simulation results, experiments were performed with installation of final version of combustion bowl geometry in C1-8 mode which is one of main test cycle to meet Tier4 final emission regulations. Finally, Tier4 final emission regulations were met with changing the piston bowl geometry without DPF.