Nanocrystalline Ni powders were successfully fabricated by mechanically milling at cryogenic temperature (cryomilling) with 1 wt.%Y2O3 particles. The experimental results have shown that the Ni grain size is reduced to 25 nmafter 2 h of cryomilling in the presence of the Y2O3 particles. The cryomilled Ni/Y2O3 powders can maintain their nanocrystalline structure up to 900℃, or 62% of the melting point of Ni. A bulk nanocrystalline Ni/Y2O3 material with a thermally stable grain size of approximately 100 nm was produced by cryomilling, cold isostatic pressing, followed by hot isostatic pressing. The microhardness of bulk nanocrystalline Ni/1wt.%Y2O3 is 315 DPH, which is two times as high as that of conventional Ni.