Taking notice of the accelerating effect of oxygen on sintering of UO₂, this study was undertaken to investigate effects on sintering characteristics of slightly oxidizing atmospheres with use of CO₂/CO mixed gas and other sintering conditions. When the sintering atmosphere for UO₂powder is made slightly oxidizing by adjusting the CO₂/CO ratio of the mixed gas, the sintering temperature can be lowered as much as 600℃ or more in comparison with sintering under the conventional reducing atmosphere. Sintered pellets having the O/U ratio of 2.01 can be readily obtained by reducing at 1100℃ for one hour pellets sintered under a slightly oxidizing atmosphere. Sintered pellets having the density of 96% of the theoretical density can be readily obtained by sintering for a short period under a slightly oxidizing atmosphere followed by a reducing treatment.