Abstract: The causes of fracture occurring in the assembling process of an H13 steel injection molding hot runner nozzle were analyzed by macroscopic test, chemical composition analysis, hardness test, metallographic examination and so on. The results show that the fracture of the hot runner nozzle was mainly caused by the high nitrogen potential in the nitrided layer when it was nitrocarburized which led to the cracking and spalling of the screw surface and resulted in the brittle fracture of the part during assembling. At the same time, the fillet radius of the bottom groove of inner hole screw was very small and the stress concentration degree was greatly increased, which also increased the cracking tendency of the parts. The heating zone on outer surface of the hot runner nozzle was not subjected to stress relief annealing after welding process, which greatly reduced the strength and toughness of the material and further increased the brittle cracking tendency of the parts. The design hardness of the hot runner nozzle was rather high, which could increase the strength, but the toughness of the material was also decreased. Finally, improvement measures were put forward according to the fracture reasons.