The isothermal crystallization kinetics and effect of crystallinity on the optical properties of cerium dioxide (CeO 2 ) nanopowders synthesized using a coprecipitation route at 293K and pH 9 were investigated using X-ray diffraction, transmission electron microscopy, selected area electron diffraction, and ultraviolet–visible absorption spectrophotometry. The activation energy of CeO 2 crystallization from dried cerium dioxide precursor powders by isothermal method of 64.1±3.24kJ/mol was obtained. The average value of the growth morphology parameter (n) is 1.94, meaning that two-dimensional growth with plate-like morphology was the primary mechanism of CeO 2 crystallization from cerium dioxide precursor powders. The indirect band gap energy (E i ) of CeO 2 decreased from 3.03eV to 2.83eV when the crystallinity increased from 18% to 82%, and the direct band gap energy (E d ) of CeO 2 also decreased from 3.76eV to 3.64eV.