Abstract: This paper studies the effects of sintering temperature and thorium content On the density of（Th,U）O₂ pellets. The activation energy for sintering（Th,U）O₂ pellets were calculated, the movement and transplantation were analyzed by using Scanning Electron Microscope（SEM）. The results show that the density of（Th,U）O₂ pellets decrease as the proportion of thorium increase when sintering temperature is constant. The density of pellets increase as the sintering temperature increase. The activation energy for sintering（Th,U）O₂ pellets increase as the proportion of thorium increase. When the proportion of thorium were 20%, 50%,80%, the activation energy for sintering（Th,U）O₂ pellets were 277.65 k J/mol, 300.70 k J/mol,380.99 k J/mol respectively. The pores in the（Th,U）O₂ pellets which the proportion of thoriumwas 20% were mainly at the junction of grain boundary, and most of the pores were spherical.