Bolt/double nuts assemblies are used for preventing the self loosening when the clamp force is relatively low. In general, the fatigue load acting on the bolt increases in the bolted joint with low clamp force due to the separation at the contact plane. However, the effect of the locking force on the fatigue strength of the bolt has not yet been clarified. This study aims to evaluate the fatigue strength of a bolt in bolt/double nuts assemblies quantitatively. Fatigue tests were performed for bolt/double nuts assemblies by taking the locking force and the nut height as parameters. FE stress analysis was also performed to predict the fatigue strength using Ishibashi's and Yoshimoto's hypotheses concerning the relationship between the local stress and the fatigue strength of the material. The results show that the locking force has the effect to decrease the local mean stress of the fatigue loading on the first thread root of the bolt mating with the lower nut, and it leads to the increase of the fatigue strength of the bolt expressed by the nominal stress.