Unirradiated UO2 pellets were pulverized by oxidation in air at 40$0^{circ}C$, and the oxidized powders were reduced in H2 and CO atmospheres at $600^{circ}C$. During the oxidation of UO2 at 40$0^{circ}C$, intergranular cracks which caused the spallation were mainly developed by the volume contraction due to the formation of intermediate phase (U4O9 or U3O7). As oxidation proceeded, U3O8 finally formed. As the oxidation/reduction cycles were repeated, the powder surface became coarser, specific surface area was increased and average particle size was decreased. The sintered densities of the powder were increased by the oxidation/reduction cycle due to the characteristic changes of the powder.