In the current research, a CFD simulation has been carried out to investigate the simultaneous effects of injection pressure and Exhaust Gas Recirculation (EGR) on the engine performance and the amount of pollutant emissions in a modern High Speed Direct Injection (HSDI) diesel engine. For this purpose, the computational results have been firstly compared to the measured data and a good agreement has been achieved in regard to predicting the in-cylinder pressure and the amount of NOx and soot emissions. Then, the effects of injection pressure has been studied at the various EGR rates. The results show, in the case of a constant injected fuel and without using the EGR, decreasing the nozzle diameter causes increase of Indicated Mean Effective Pressure (IMEP) and reduction of Indicate Specific Fuel Consumption (ISFC). However, the amount of NOx emission has been largely increased and the amount of soot emissions has been decreased. By applying the different percentage of EGR rates, a potential to decrease the amount of NOx emission have been provided while the engine operating conditions (including the ISFC and IMEP) remain constant. However, the increase of soot emissions should be considered as a negative parameter in this case.