Inconel X-750 is an age-hardened Ni-based superalloy with high mechanical strength and creep resistance. The X-750 alloy is extensively used in the cores of reactors, such as spacers in CANada Deuterium Uranium (CANDU) fuel channels. The recent mechanical tests on the ex-service Inconel X-750 spacers indicate significant embrittlement and reduced load carrying capacity compared to as installed condition. The primary degradation mechanism remains unclear, and thus provides the focus of this investigation. Heavy-ion irradiation was employed as an emulator for neutron irradiation to explore the microstructural evolution and mechanical property degradation in X-750 Ni-based superalloy. The ion-irradiation has been conducted at different temperatures and up to different doses. In addition, helium-implantation prior to heavy-ion irradiation was used to investigate the effect of helium on microstructural changes. The discussion of the microstructural evolution is focused on characterization of irradiation-induced defects, including dislocation loops and cavities along with examination of the stability of strengthening phase gamma'-precipitates. A major contribution of this work is to utilize a focused ion beam (FIB) and transmission electron microscopy (TEM) to perform precise defect characterization. The microstructure is correlated to the mechanical properties obtained via nano-hardness test on irradiated materials. In order to estimate the individual contribution of defects in radiation-induced hardening, three different obstacle-hardening models have been applied to fit TEM-obtained microstructural data. The superposition of contributions is made in each model and compared with nano-scale experimental results. This approach is unique to the literature since it demonstrates both the individual and the combined effects of the microstructural features on mechanical behavior. Furthermore, the effect of gamma-prime instability on mechanical properties of irradiated X-750 was investigated and the strength softening raised from disordering and also dissolution of gamma-prime was evaluated.