Dense nano-sized UO2+x pellets are synthesized by spark plasma sintering with controlled stoichiometries (UO2.03 and UO2.11) and grain sizes (similar to 100 nm), and subsequently isothermally annealed to study their effects on grain growth kinetics and microstructure stability. The grain growth kinetics is determined and analyzed focusing on the interaction between grain boundary migration, pore growth, and coalescence. Grains grow much bigger in nano-sized UO2.11 than UO2.03 upon thermal annealing, consistent with the fact that hyper-stoichiometric UO2+x is beneficial for sintering due to enhanced U ion diffusion from excessive O ion interstitials. The activation energies of the grain growth for UO2.03 and UO2.11 are determined as similar to 1.0 and similar to 2.0 eV, respectively. As compared with the micrometer-sized UO2 in which volumetric diffusion dominates the grain coarsening with an activation energy of similar to 3.0 eV, the enhanced grain growth kinetics in nano-sized UO2+x suggests that grain boundary diffusion controls grain growth. The higher activation energy of more hyper-stoichiometric nano-sized UO2.11 may be attributed to the excessive O interstitials pinning grain boundary migration.