Abstract: Two kinds of seal houses for control rod drive mechanisms which have different thickness of the lower seal ring was analyzed for its stress and fatigue by finite elemet method.In the fatigue compution,all the transitions were grouped into several groups,and then the elastoplastic strain correction factor was modified by analyzing thermal and mechanical load separately referring the rules of RCC-M 2002.The results show that the structure with thicker seal ring behaves more safely than the other one except in the second condition.Meanwhile,the amplify of the primary and secondary stress as well as fatigue usage factor can be reduced by regrouping the transients.The precision of fatigue usage factor can be elevated using modified Ke when the amplify of the primary and secondary stress is large to some extent produced by both thermal and mechanical loads.