Abstract: The objective of this study is to simulate combustion process, pollutant formation and flow field in the combustion chamber of a DI diesel engine converted to work as a dual fuel (Diesel/Natural gas) engine. Effect of natural gas equivalence ratio, in constant diesel pilot injection and piston bowl shape, on the combustion process, pollutant formation and flow field are investigated in 5 cases defined. An eddy break-up combustion model and a diesel auto-ignition model were implemented to simulate the ignition and combustion processes. Experiments were performed on OM_355 DI diesel engine to validate the simulations. Results show that there have been good agreements between experiments and the CFD calculations.