Nanosized cerium dioxide (CeO 2 ) powders have been synthesized using coprecipitation methods and cerium nitrate hexahydrate (Ce(NO 3 ) 3 ·6H 2 O) as the starting material. The growth and optical properties of nanosized CeO 2 powders were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), nano-beam electron diffraction (NBED), high resolution TEM (HRTEM), and ultraviolet–visible (UV–vis) absorption spectrophotometry. The XRD result shows that the dried CeO 2 precursor powders (both before and after calcination at various temperatures and times) contained a single crystalline phase of CeO 2 . In the dried precursor powders, the crystallites of CeO 2 measured 10.4nm and 66.8nm before and after calcination at 1273K for 240min, respectively. The indirect band gap energy (Ei) of CeO 2 decreased from 3.03eV to 2.68eV as the crystallite size increased from 10.4nm to 66.8nm, whereas the direct band gap energy (Ed) of CeO 2 also decreased from 3.79eV to 3.38eV.