This paper presents an experimental investigation of the influence of fuel density and fuel viscosity on the flow characteristics generated from a high pressure Diesel injector, equipped with conical orifices. For this purpose, mass flow rate measurements were performed with nine different fuels over operating conditions from 30 to 180 MPa of injection pressure and from 1 to 9 MPa of back pressure. Fuel viscosity was varied from 0.6 to 7 mm2/s and fuel density from 683 to 876 kg/m3 at the operating temperature. Fuel viscosity induces a decrease of up to 10% in the discharge coefficient at low pressure difference but for higher pressure difference, fuel density remains the only fuel property driving the mass flow rate. The discharge coefficient is also a function of the pressure difference at any given Reynolds number under non-cavitating conditions. A correlation is suggested to estimate the discharge coefficient taking into account the fuel properties and the operating conditions.Highlights► Effect of fuel properties on the injection rate in a Diesel injector. ► Increasing viscosity decreases the discharge coefficient at low injection pressure. ► Discharge coefficient increases with the pressure difference. ► Increasing density increases the mass flow rate. ► A new correlation is proposed.