Abstract: One screw of a lubropump fractured during strengthening reliability test. The fracture mode and cause were studied by macro and micro observa-tion, metallographic examination, and hardness measurement. The results show that the failure mode of the screw is fatigue fractur, which was mainly caused by the poor contacted state between the screw cap and the surface near the hole. The effect of the poor contact state on stress distribution was further studied by finite element method (FEM). The computation results shows local loading could change stress distribution of screw and obviously increase the maximum stress of the bottom of thread at the same side with local loading, and it could be concluded that the screw fracture was mainly caused by the weld point con-tacting the cap of screw.