Nanocrystalline cerium dioxide (CeO2) has been successfully synthesized by mechanochemical technique at 12, 24, 36, 48 and 60 hours milling times. The starting materials, hydrated cerium carbonate and sodium hydroxide, were mixed in a weight ratio of 4:1 and were milled in a planetary ball mill with ball to powder ratio of 10:1. The high energy impact forces provided by the milling media caused collision of starting materials and allowed the chemical reaction to occur thus produced nanocrystalline cerium dioxide. The products were characterized using a battery of characterization methods, including thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and gas adsorption-desorption measurement. The nanocrystalline CeO2 with 6.7 nm of crystallite size and specific surface area of 66.66 m2/g was obtained when the sample was milled for 60 hrs and they annealed in air at 350 oC. The result showed that the crystallinity of nanocrystalline CeO2 decreased with increasing milling time.