To promote advanced combustion strategies complying with stringent emission regulations of CI engines, computational models have to accurately predict the injector inner flow and cavitation development in the nozzle. This paper describes a coupled 1D/2D/3D modeling technique for the simulation of fuel flow and nozzle cavitation in diesel injection systems.The new technique comprises 1D fuel flow, 2D multi-body dynamics and 3D modeling of nozzle inner flow using a multi-fluid method. The 1D/2D model of the common rail injector is created with AVL software Boost-Hydsim. The computational mesh including the nozzle sac with spray holes is generated with AVL meshing tool Fame. 3D multi-phase calculations are performed with AVL software FIRE. The co-simulation procedure is controlled by Boost-Hydsim. Initially Hydsim performs a standalone 1D simulation until the needle lift reaches a prescribed tolerance (typically 2 to 5 μm). From this time instant the 1D/2D/3D co-simulation with the FIRE multiphase solver is started. During the co-simulation process Boost-Hydsim transmits to FIRE the displacement vector of the needle tip and fuel pressure and temperature at the nozzle interface. Based on this data, FIRE moves the computational mesh, adjusts boundary conditions, computes the time step and sends back to Boost-Hydsim the hydraulic force vector acting on the needle tip, sac pressure and mass flow rate through the needle seat and nozzle holes.