Nanocrystalline UO2+x powders are prepared by high-energy ball milling and subsequently consolidated into dense fuel pellets (>95% of theoretical density) under high pressure (750MPa) by spark plasma sintering at low sintering temperatures (600 degrees C-700 degrees C). The grain size achieved in the dense nano-ceramic pellets varies within 60-160nm as controlled by sintering temperature and duration. The sintered fuel pellets are single phase UO2+x with hyper-stoichiometric compositions as derived by X-ray diffraction, and micro-Raman measurements indicate that random oxygen interstitials and Willis clusters dominate the single phase nano-sized oxide pellets of UO2.03 and UO2.11, respectively. The thermal conductivities of the densified nano-sized oxide fuel pellets are measured by laser flash, and the fuel stoichiometry displays a dominant effect in controlling thermal transport properties. A reduction in thermal conductivity is also observed for the dense nano-sized pellets as compared with micron-sized counterparts reported in the literature. The correlation among the SPS sintering parametersmicrostructure controlproperties is established, and the nano-sized UO2+x pellets with controlled microstructure can serve as the model systems for fundamental understandings of fuel behaviors and obtaining critical experimental data for multi-physics MARMOT model validation.