A computational study focused on the inner nozzle flow and cavitation phenomena has been reported in this paper in order to investigate the two most common types of diesel injector nozzles at the present: microsac and valve covered orifice (VCO). The geometrical differences among both types of nozzles are mainly located at the needle seat, upstream of the discharge orifices. In the case of microsac nozzles there is a small volume upstream of the discharge orifices which is not present in VCO nozzles. Due to these geometrical differences among both type of nozzles, differences in the inner flow and the cavitation development have been found and analysed in this research. For the study, two cylindrical nozzles with six orifices and the same outlet diameter have been experimentally characterized in terms of mass flow rate. These measurements have been used to validate the CFD results obtained with the code OpenFOAM used for the analysis of the internal nozzle flow. For the simulations, two meshes that reproduce the microsac and VCO nozzles seat geometry while keeping the same geometry at the orifices have been built. The simulations have been carried out with a code previously validated and able to simulate cavitation phenomena using a homogeneous equilibrium model (HEM) and with RANS approach for the turbulence modelling (RNG k–ε).