Abstract: M1.0 screws fractured during the process of tightening. To find out the fracture cause, a fractured screw was analyzed by means of scanning electron microscopy(SEM), chemical composition analysis, metallurgical structure examination and Vickers hardness testing. And some comparative analyses were also carried out between the fractured screw and the normal screws having passed the twist test. The results show that there was composition segregation at the core of the fractured screw, which resulted in zonal structure after heat treatment. The core metallurgical structure was pearlite and ferrite in zonal distribution. The zonal structure led the strength of the screw to decrease. As a result, the screw fractured when the test torque was lower than the designed value. The raw materials for screws should be examined to avoid such failure cases.