Abstract: The mechanism of diffusion of ion and chemical transformation rule on the formation of solid solution for UO₂-ZrO₂ were investigated by XRD patterns and SEM micrographs. The results show that ZrO₂ with UO₂ diffuses and reacts more easily than UO₂ with ZrO₂. The phase transformation of ZrO₂ from monoclinic to tetragonal enhances the formation of UO₂ solid solution. The structural evolution of pellet depends on the process of the atoms diffusing across the (U_xZr_1-x)O₂and (Zr_yU_1-y)O₂ layers and the reaction at the interface. The differences of cation radii and crystal structure result in that ZrO₂ can dissolve into UO₂ more than UO₂ into ZrO2, which leads the formation of solid solution mainly depends on the growth of UO₂ solid solution, and the ZrO₂ solid solution decreases gradually during the sintering process. The formation of solid solution is mainly the substitution of Zr⁴⁺ for U⁴⁺ via the schottky defect.