The shrinkage behaviour of UO₂ has been studied using a dilatometer in various atmospheres of Ar, Ar–8%H₂,vacuum, CO₂, commercial N2and N_{2 +} 1000 ppm of O2. The onset of shrinkage occurs at around 300–400⁰C lower in oxidizing atmospheres such as CO₂, commercial N₂ and N_{2 +}1000 ppm O₂ compared to that in reducing or inert atmospheres. Shrinkage behaviour of UO₂ is almost identical in Ar, Ar–8%H₂and vacuum. The shrinkage in N₂+1000 ppm O₂ begins at a lower temperature than that in the commercial N₂. The mechanism of sintering in the reducing, inert and vacuum atmospheres is explained by diffusion of uranium vacancies and that in the oxidizing atmospheres bycluster formation.