The scope of this work is to investigate the simultaneous effects of injection pressure and Exhaust Gas Recirculation (EGR) on mixture formation and engine performance in a High Speed Direct Injection (HSDI) diesel engine. For this, the computational results have been firstly compared to the measured data and a good agreement has been achieved in order to predict the in-cylinder pressure, heat release rate and the amount of NOx and soot emissions. Then, various injection pressures have been studied to explore its benefits to achieve the low exhaust emission at different EGR rates. The results show, while no EGR has been applied, decreasing the nozzle diameter causes the reduction of Indicate Specific Fuel Consumption (ISFC) with an increase in Indicated Mean Effective Pressure (IMEP). In addition, this strategy results to better air-fuel mixing, a faster combustion process, a considerable reduction of soot emissions but at the same time to a significant increase of NOx emission. By applying the different percentages of EGR rates, a potential to decrease the amount of NOx emission have been provided at high injection pressure while the other engine operating conditions (including the ISFC and IMEP) remain nearly constant. Nevertheless, the increase of soot emissions should be still considered as a negative effect in this case.