Abstract: Large grain UO₂ fuel kernels are expected to reduce fission gas release which can achieve extended burn up and life of reactor. The UO₂ fuel kernels of HTGR were prepared by sol-gel method, and doped with Al(NO₃)₃•9H₂O to increase their grain sizes. Microstructure of UO₂ fuel kernels of HTGR and the relations of the O ions released by the dissociation of Al(OH)₃ and the diffusion coefficient of U ions were studied. The effect of dopant content on the grain sizes of UO₂ fuel kernels was investigated. It indicates that the grain sizes are increased from 18 μm to 30 μm. The sintering mechanism of UO₂ fuel kernels by the addition of Al(NO₃)₃•9H₂O shows that the grain sizes of UO₂ are affected by the diffusion of vacancy.