Abstract: Oxidation mechanism and kinetics in static air at 513 K of uranium dioxide powder was investigated on purpose to obtain surface pre-oxidation treated active uranium dioxide powder whose O/U ratio in outer layer was approximately 2.25 while whose phase in core was still uranium dioxide so as to carry out low temperature sintering test. Oxidation process of uranium dioxide powder oxidized at 513 K is divided into the earlier period and the later period. The limitation of absolute increased mass ratio of uranium dioxide powder at 513 K in the later period is 0.489%. The reaction rate is controlled by interface chemical reaction in the earlier period and by diffusion in the later period. Oxidation mechanism of uranium dioxide powder oxidized at 513 K in the earlier period conforms to scaled core model. The functional relationship between increased mass in unit area of single uranium dioxide particle and oxidation time t can be expressed by equation w/Sn＝0.104 1t－ 2.146 67, oxidation rate constant kc in the earlier period of uranium dioxide powder is 3.49×10⁶exp(－73 919.39/RT).