Abstract: Based on fatigue experiment and finite element method (FEM), the effect of radial gap on the vibration fatigue was researched. The test results show that crack with a higher stress tend to originate at the weld toe while for the case of lower stress failures tend to occur at the root. The root failure is that the crack originates at the inside of weld roots and propagates to the outside surface of specimen on the socket side of the weld, the crack of toe failure initiates at the outside surface of the specimen near the pipe side toe of the weld and propagates to the inside surface. The radial gap of runout is higher than that of failure. The effects of radial gap on weld root and weld toe were also investigated by FEM. Results show that the radial gap can significantly reduce the stresses of weld root and weld toe, which decrease 14% and 1.4% with the width of radial gap from 0.1 mm to 0.4 mm. Further, the radial gap can redistribute the stress in the weld root and weld toe, which is beneficial to the fatigue life.