Fuel injection nozzles play a crucial role when it comes to operation, efficiency, and emissions from diesel engines. In a nozzle, at sharp edges due to the abrupt change in flow direction, the fluid flow streamlines are being contracted. The f luid is accelerated in this section thus reducing the pressure creating ideal conditions for cavitation. Cavitation number can characterize the conditions required to satisfy cavitation. Even though a considerable number of researchers have observed cavitation in injectors, a strong emphasis has not been given to the material degradation taking place within the injector sac volume and the needle seating. The current investigation is based on observations of a real-size injector, which was subjected to cavitation and comparison of its topology features with that of a new one. Material composition and material parameters were analysed. Cavitation erosion resistance of the material was also determined. Experiments were carried out to determine the mass flow rate of four injector nozzles that have done 40,000 km, 81,000 km, 120,000 km with a new one. Observation of cavitation erosion in the needle seat area on a real-size diesel fuel injector was compared with that of a new one. Comparing the experimental observations with evidence from literature it was revealed that a severe cavitation erosion is occurring in the needle seat area within the sac volume of injectors. Further literature supports this observation as there were cases where injectors have failed due to cavitation.