In the W/O microemulsion system of cetyltrim ethylamine bromide (CTAB) + butanol / cyclohexane / water solution, the microemulsion area was measured and cerium oxide nanoparticles were prepared according to the principle by two-phase liquid–liquid method. The samples were analyzed by means of transmission electron microscope, X-ray diffraction, thermogravimetric-differential scanning calorimetry, and infrared. And their morphology, phase, components were investigated. The results made clear that nanometer cerium oxide could be prepared in the measured area of microemulsion and it is an effective way to gain the well dispersed and uniformly distributed nanometer cerium oxide grains with the size of about 3 nm.