This paper focuses on the study of heat release rate (HRR) and in-cylinder pressure on the homogeneous charge compression ignition (HCCI) process in a modified diesel engine (Deuzt Diter FL1 906). A zero-dimensional thermodynamic model was used to calculate the HRR. The effects of inlet temperature, compression ratio, and exhaust gas recirculation (EGR) on the HRR and engine-out emissions have been investigated experimentally at speed range (1200-2400 RPM), where the start of injection has been fixed at 45 degrees before top dead center (BTDC). The EGR, a well-known and widely used method to depress NOx emission in diesel engines and in HCCI combustion mode, is also used as a basic method to control the ignition timing and burning rate. The total combustion duration was prolonged with an increase of EGR, and the start of combustion was delayed. The HCCI combustion mode shows a HRR clearly different from the HRR correspondent to conventional diesel combustion as it is approached in Wiebe's function of two modes.