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[en] Dense nanocrystalline and microcrystalline UO2 samples with controlled grain structure and stoichiometry were prepared by high energy ball milling and spark plasma sintering (SPS). Nano-indentation and micro-indentation testing were performed at different temperatures of 25 °C, 300 °C, and 600 °C in order to study the mechanical properties of the sintered fuels as functions of grain structure and temperature. Nanocrystalline UO2 display higher hardness than microcrystalline counterpart, consistent with the Hall-Petch strengthening mechanism. Greater Young's modulus and fracture toughness are also identified for the nanocrystalline UO2, and hardness and Young's modulus decrease with temperature, suggesting better ductility of oxide fuels at high temperature and small length scale. Hyper-stoichiometric UO2 specimen displays higher hardness and fracture toughness than stoichiometric UO2, due to the impediment of the crack propagation by the oxygen interstitial atoms. These results are useful in understanding the mechanical properties of the high burn-up structure (HBS) formed in nuclear fuels during reactor operation, and also provide critical experimental data as the input for the development and validation of the MARMOT fracture model of nuclear fuels.