Nanocrystalline Eu 3+-doped CeO2, CeO2,Sm 3+-doped CeO2, and Li +,Eu 3+- codoped CeO2 samples were prepared through a sol - gel process. The structure and the optical properties of the samples were characterized by X-ray diffraction, diffuse reflection spectra, and photoluminescence spectra. No luminescence was observed for nanocrystalline CeO2. The systematic investigation shows that the broad band in the excitation spectrum of CeO 2:Eu 3+ comes from the charge transfer (CT) transition from O 2- to Ce 4+, not from the oxygen vacancy, or from the CT of O 2- to Eu 3+. Upon increasing the fired temperature from 600 to 800 °C, the excitation spectrum shifts to lower energy. With increasing concentrations of Eu 3+ up to 1% in CeO 2, red shifts of the excitation spectra are observed; however, when the concentration of Eu 3+ increases to 5% and 10%, blue shifts occur. The emission spectrum shows that the symmetry of the Eu 3+ site becomes lower with increasing Eu 3+. Based on the dielectric theory of complex crystals, the environmental factor (he) and the dielectric definition of average energy gap around the centers of Eu 3+ are calculated. The reasons for the shifts of the excitation spectra are discussed in detail.