New future emission regulations for HD Diesel engines in Europe will require a further reduction in NOx and particulate emission levels compared with nowadays values.The work presented here is an analysis of the EGR influence on the DI Diesel combustion process, based upon a parametric study combining cooled EGR with intake pressure increase. The independent variation of EGR rate and intake pressure allows to analyse the influence of the oxygen concentration and the equivalent fuel/air ratio on the combustion process in an independent way. Tests were performed with a 1.8 litre single-cylinder engine. The study combines performance and emissions measurements together with heat release law analysis. Five representative operative conditions from the European Steady - state test Cycle (ESC) have been considered. For each one, EGR rate (from 0 to 30%) and intake pressure (from 0 to 0.03 MPa over nominal pressure) have been varied independently. The presented results allow to identify and to quantify some aspects of the influence of both parameters on the combustion characteristic time and on the pollutant formation.