[en] This study aims at experimental and theoretical elucidation on materials deterioration such as effect of microstructures formed by point defects occurred by radiation and point defects on deformation and fracture of the materials and at following of its control method. In 1998 fiscal year, in order to obtain a mechanism affected on low cycle fatigue fracture of the materials by dynamic radiation damage phenomenon under light ion radiation, some developing processes of stress and strain distribution at proximity of a notch end with preliminary cracks were investigated on fatigue of the 316 steels at 60 centigrade, under and after irradiation and at no irradiation already reported on some extreme radiation-induced deformations by using the finite element method. As a result, under a condition forming extreme radiation-induced deformation like under radiation at 60 centigrade, it was formed that change of stress and strain distribution accompanied with increase of repeated stress cycling numbers was controlled. This showed that the radiation-induced deformation acted to delay accumulation process of plastic deformation in fatigue. At an actual fatigue test under irradiation, it was already found that fatigue life increased and that a result agreeing with the experimental result in spite of analysis on only accumulation process in plastic deformation was obtained by this study. However, such effect through the radiation-induced deformation is only a part of dynamic effects under irradiation, an effect due to non-continuous plastic deformation such as formation of dynamic point defect in a transfer channel and reaction and removal due to movable transfer was also thought to be contributed to elongation of the fatigue life. (G.K.)