Abstract: The point defect model (PDM) of hyper-stoichiometry uranium dioxide was built for studying the effective of defects to sintering. According to PDM, the reaction equations and the concentrations of defects were found out. The concentrations of defects are dramatically different with the change of partial pressure of oxygen (p_O2) or excess oxygen ions (x in UO_2+x). The PDM is used to calculate diffusion coefficient of uranium ions and sintering density of UO_2+x pellets, the experimental results agree the calculated results quite well. PDM explains the mechanism of low temperature sintering to UO_2+x powder.