The nickel-based alloy UNS N06625 has been selected as primary materials for critical components, such as steam generators and internal structures of fourth-generation reactors, due to their exceptional high-temperature performance. However, prolonged exposure to high temperatures and irradiation can degrade their mechanical properties, posing significant risks to reactor safety. Thus, understanding the mechanical behavior of nickel-based alloys under such conditions is of critical importance. This study reveals the effects of temperature and irradiation on the radiation resistance of UNS N06625 using ion irradiation and nanoindentation techniques. Specifically, 3.5 MeV Fe ion irradiation experiments were conducted at room temperature, 300 °C, 600 °C and 750 °C. At each temperature, three samples were irradiated to surface radiation damage levels of 0.1 dpa, 1.0 dpa and 3.0 dpa. Nanoindentation tests were subsequently performed on the samples, both before and after irradiation, with an indentation depth of 1000 nm. The results showed that irradiation increased the nanoindentation hardness of the samples to varying degrees. This study provides a theoretical basis for the engineering application of UNS N06615 alloys in reactors.