Abstract: The radial fracture of DG20 steel hook screw of a portal slewing crane happened after less than one year service on the wharf in a company. The fracture reasons of the hook screw were analyzed by means of chemical composition analysis, macro analysis, SEM analysis, metallographic examination, etc. The results show that the fracture of the hook screw was one-way bending fatigue fracture, and the fracture originated from the micro cracks at the root of the thread. Improper operation at the site resulted in serious unidirectional stress concentration at the thread root of the hook screw, which caused the crack initiation and continuous expansion and finally resulted in the fracture failure.