An advanced turbulence modeling using Large Eddy Simulation (LES) has been employed for studying diesel engine flow and its effects on combustion process and amount of pollutant emissions in a DI Diesel engine. An improved version of the Extended Coherent Flame Model combustion model (ECFM-3Z) coupled with advanced models for NOx and soot formation has been applied for CFD simulation. The model performance was assessed by comparison of the calculation results with corresponding experimental data. Very good agreement of calculated and measured in-cylinder pressure, heat release rate as well as pollutant formation trends were obtained. The simulation results was further compared with those obtained by traditional Reynolds-averaged Navier-Stokes model (RANS) at three different mesh resolutions. It was concluded that sensivity of LES approach to geometric details is affected by increasing resolution as compared to existing RANS. The results showed emissions trend predicted by LES matched experimental measurement better than RANS and LES simulation can provide additional accuracy and details of flow structure than RANS. These made the LES approach more suited for investigating detailed flow structures in complex time dependent engine flows.