Wear and deformation occur in the body seat of diesel fuel injector nozzles, thereby affecting fuel spray behaviors and injection characteristics. In this study, the effects of body seat temperature, test duration, temperature increase duration, tempering temperature, and tempering duration on the nozzle body seat are investigated using an injector testing device. Surface roughness measurements using a nano search microscope and hardness measurements of the nozzle body seat are performed to investigate the textural and physical characteristics of the nozzle body seat. In addition, optical measurements via optical microscopy and field emission scanning electron microscopy are performed to investigate the topographical information and microstructure of the nozzle body seat. The results show that the increase in wear and deformation is related to the higher test temperature and duration. Additionally, the tempering technique can suppress the deformation of the nozzle body seat.