Because the deformation of double hull Afra-Max tanker is larger than that of ordinary single hull tanker, it is considered that the additional stress of secondary structural member induced by deformation of main structural member becomes large. Then FEM analysis of secondary structural member is carried out. And the mechanism of additional stress due to deformation of main structure is presented. According to the calculated result or proposed mechanism, the additional stress of stiffener attached to transverse web becomes large when the depth of double hull is small. The stress amplitude due to wave becomes large at longitudinal side frame crossing bulkhead as same as ordinary single hull tanker. The characteristics of stress due to the change of structural detail of secondary member are also presented in this paper.