A combination of transmission electron microscopy (TEM) and laser-assisted three-dimensional atom probe (3DAP) is employed to study the nanostructure of Dy-doped CeO2, which is a promising ionic conductor. Segregation of Dy atoms at grain boundaries is observed by electron energy loss spectroscopy (EELS) elemental maps. The segregation is checked and confirmed by laser-assisted 3DAP. Moreover, the enrichment of Dy and deficiency of Ce at grain boundaries are quantitatively identified by the 3DAP concentration profile. Data such as these are significant to optimize the electrical conductive property in rare-earth doped Ceria.