In this paper the results of experimental irradiation studies on mechanical properties, grain boundary deformation, and substructural formation in nickel specimens irradiated in the temperature region of high-temperature radiation embrittlement (HTRE) appearance are presented. Flat nickel specimens with the work surface 0.3 by 3.0 by 10 mm were electron-irradiated to the energy 224 MeV at 100°C to the dose 1 × 1021 e/cm2 (0.2 dpa). The amount of helium in the irradiated specimens was approximately 2.5 × 10-3 at%. Those specimens that had been irradiated to the dose 2.5 × 1018 e/cm2 had the helium content of approximately 10-5 at%. Some specimens were α-particle irradiated until reaching helium concentration of 10-5 at %. The α-particle energy was 30 MeV, generating about 10-8 dpa of damage. The mechanical tension tests were carried out in vacuum 5.0 × 10.0-5 torr (655 Pa) at a rate of 3.0 × 10-3 s-1.