Developing novel materials for nuclear reactors is a crucial research task. Due to the harsh conditions in the reactors, core materials must possess exceptional high-temperature properties, including high strength, ductility, corrosion resistance, and irradiation resistance. Additionally, the low neutron absorption cross-section and the neutron activation are also important considerations. It is widely acknowledged that the choice of core material for a typical space nuclear reactor is primarily determined by the operation temperature. Generally, with increasing the designed operation temperature of reactor, 316 stainless steel, nickel-based superalloys, oxide-dispersion-strengthened (ODS) steel, refractory metals, and SiC ceramics are preferred in order. Besides, the high entropy alloys attract extensive attention for nuclear applications. This review summaries the evolution of mechanical properties of different material systems during irradiation process, providing guidance for the further research in irradiation resistance.