The method of molecular dynamics is used to study phase transitions and ion transport phenomena in uranium dioxide nanocrystals. The temperature dependences of uranium and oxygen diffusion coefficients in the range from 2280 to 3950 K are calculated separately for surface, near-surface and bulk regions of the crystals. On these dependences we have distinguished temperature intervals, which correspond to crystalline phase, superionic state and melt. The activation energies of diffusion and pre-exponential factors for these intervals are determined. The obtained results are compared with experimental data and calculations of other authors. (C) 2007 Elsevier B.V. All rights reserved.