Loosening-fatigue tests under transverse vibration have been performed to reveal the effect of bolt size in a bolted joint subjected to transverse vibration. Commercial hexagon head bolts in three sizes; M8, M10 and M12, were used in the experiment. Size effect has been verified by means of estimating the nominal stresses at the first thread root using test bolts with attached strain gages. Differences in fatigue characteristics between a bolted joint and a bolt/nut assembly have also been investigated. The results showed that the actual fatigue limit decreased with an increase in bolt size although the larger size bolt has a higher apparent fatigue limit, the threshold of the amplitude of transverse vibration force. The decreasing ratio was larger than the decreasing ratio in the axial fatigue of a bolted joint. It was confirmed that the fatigue characteristic under transverse vibration shows a marked size effect. The results also showed that the threshold of the amplitude of transverse vibration corresponding to the apparent fatigue limit of the bolt/nut assembly was higher than that of the bolted joint.