The paper concerns the classical Diesel fuel injection system pump-high pressure pipe-nozzle.The influence of the individual components of fuel injection system on the injection process and hydraulic phenomena in the fuel injection system are described in detail. The mathematical model describing all these phenomena has been solved numerically by the Runge-Kutta IV method. The second part of the paper deals with the testing of the injection system. All tests have been perfomed on a R. Bosch test stand and on the one designed by ourselves. The comparison between the computed and meeasured results shows good matching.