In this study, a low temperature sintering proces at 1000-1300℃ in CO₂ atmosphere was employed, and three green pellets with O/U ratio of 2.17, 2.19 and 2.49 were used to investigate the effect of the O/U ratio on the microstructure and density of sintered UO₂ pellets. As the O/U ratio of green pellets increased, the quantity of fine grain (i.e., smaller than 2㎛ in diameter) decreased, and the sintered density decreased. When green pellets with O/U ratio of 2.49 was sintered, the monomodal microstructure, which is mainly consisted of uniform and coarse grains (i.e., larger than 30㎛ in diameter), was obtained.