For the structural materials of Fast Breeder Reactor, the modified type 316 ; 316MN has been developed. In this steel, improvement of creep fatigue endurance was achieved by reducing carbon and adding nitrogen. In this study, with conventional SUS316 (0.05%C-0.03%N) and 316MN (0.01%C-0.07%N) produced by commercial product line, changes of creep fatigue properties at high temperature after aging, stress-aging and fatigue were investigated in consideration of microstructure change. Moreover, quantitative evaluation of damage by microstructure change during creep fatigue was tried with remaining ductility for SSRT (Slow Strain Ratetensile Test). In result, it was certified that 316MN had excellent stability of microstructure at high temperature against aging, stress-aging and fatigue. Therefore, the creep fatigue properties of 316MN were little damaged by aging and stress-aging. Furthermore, after fatigue, , the remaining ductility of 316MN was able to hold approximately three times as much as that of conventional SUS 316.
