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A series of rare earth bulks with the ultrafine nanocrystalline structure were prepared by applying an "oxygen-free" (an environ-mental oxygen concentration less than 0.5 ppm) in-situ synthesis system, where the inert-gas condensation was combined with the spark plasma sintering technology into an entirely closed system. The thermal and mechanical properties of the prepared ultrafine nanocrystalline bulks were characterized and compared with those of the raw polycrystalline bulks. It was found that the specific heat capacity, the micro-hardness, and the elastic modulus of the ultrafine nanocrystalline rare earth bulks were significantly increased from the conventional poly-crystalline bulks.