The variation of the sintered density and grain size of UO$_2$ as a function of the Li$_2$O amount and sintering atmosphere was observed. Li$_2$O enhanced the grain growth of the UO$_2$ pellet in H$_2$, but rather hindered it in $CO_2$ atmosphere. Grain size of the UO$_2$ and UO$_2$-0.1 wt%Li$_2$O pellets was, respectively, 8 $mu$m and 100 $mu$m at 168$0^{circ}C$ in the H$_2$ atmosphere, and that of each pellet was, respectively, 24 $mu$m and 17 $mu$m at the same temperature in the $CO_2$ atmosphere. As-received Li$_2$O powder, which had been composed of Li$_2$O and LiOH, was converted to the Li$_2$CO$_3$ phase after heating to 80$0^{circ}C$ in $CO_2$. On the other hand, the Li$_2$O and LiOH phases remained unchanged in H$_2$ atmosphere. In the H$_2$, the as-received Li$_2$O powder began to evaporate at about 105$0^{circ}C$ and then about 20 wt% residue was left at 150$0^{circ}C$. But, most of the Li elements evaporated at 150$0^{circ}C$ in the $CO_2$ atmosphere.