Abstract: A grade HY sucker rod in an oilfield fractured at the trasition between wrench side and external thread step during service. The fracture cause was studied by fracture morphology observation, chemical composition analysis, microstructure observation, hardness test and finite element simulation. The results show that the fracture mode of failed sucker rod was brittle fracture, and the crack source was located at the edge corners of the transition position between the wrench side and the external thread step. The cause of fracture was the content of carbon and molybdenum elements lower than that of the standard requirement and higher content of ferrite in microstructure led to insufficient hardenability and strength of sucker rods, and the transition fillet radius between the wrench side and the external thread step did not meet the standard requirement and there were pit defects on the surface of the transition position, resulting in greater stress concentration.