Austenitic stainless steel welds (ASSWs) are required to have duplex microstructure composed of austenite matrix and a certain amount of δ-ferrite to avoid hot cracking during welding in light water reactors (LWRs) application. However, due to presence of δ-ferrite, ASSWs are subjected to suffer thermal embrittlement after long term exposure at service temperature in the range of 280 to 320 oC due to phase separation by spinodal decomposition and G-phase precipitate. Since their mechanical performance is likely to be deteriorated with aging time, fundamental understanding for aging induced microstructure evolution and its effect on mechanical property is of great interest for safe operation of LWRs. Furthermore, ASSWs in reactor core internals are subjected to neutron irradiation, which could accelerate the microstructure evolution and hardening by radiation enhanced diffusion. Although a number of studies have been performed to understand the thermal ageing and irradiation embrittlement, degradation mechanisms were not fully understood yet. In this thesis, after accelerated thermal aging to simulate long term exposure at LWRs relevant condition, the deformation stability and its contribution to hardening of δ-ferrite were evaluated by nano-scale characterization of deformation behavior. Then, in order to estimate kinetics of hardening behavior, thermal aging activation energies of δ-ferrite were characterized using multi-scale mechanical property tests and those results were interpreted by effect of alloying elements and microstructural evolution. Finally, after proton irradiation to emulate neutron irradiation, the combined effect of thermal aging and irradiation was investigated and additional hardening mechanism was suggested.