The residual stress at the thread root of a bolt induced by thread rolling is one of the major factors influencing the fatigue strength, and the method (hypothesis) to predict the fatigue strength has been investigated. However, the effect of the residual stress has not been verified quantitatively since there are many other variables affecting the fatigue strength data. This study aims to clarify the hypothesis on the fatigue strength taking into account the effect of residual stress at the thread root. Fatigue tests have been carried out for the heat-treated test specimens after or before thread rolling process. Experimental data show that the fatigue strength of bolts thread-rolled after heat treatment is about 100% higher compared to bolts thread-rolled before heat treatment in the low mean stress condition, and the difference decreases as the increase of mean stress of the fatigue loading. These results show good agreement with the hypothesis presented assuming that the compressive residual stress is 1200N/mm^2 to 1600N/mm^2 level for 8.8 and 12.9 bolts, respectively.
