The crystal size dependence of the bulk modulus of nano-crystalline uranium dioxide is investigated by means of in-situ high pressure x-ray diffraction. Samples with average particle sizes of 4, 6 and 34 nm as measured from high resolution TEM images were studied up to 27 GPa revealing that all three nano-crystalline UO2 samples show a drastic decrease of bulk modulus with decreasing particle size. The influence of choosing the right EOS to fit the compression data is also discussed. ; JRC.E.6-Actinide research