Nanometer-sized CeO2 particles with monodispersity and narrow size distribution were prepared by microemulsion method in CTAB/butanol/cyclohexane/water solution. Micro- emulsion zones of distilled water, Ce(NO3)3 solution and ammonia were measured, respectively. And a proper micro- emulsion system to prepare CeO2 nanoparticles was confirmed. According to the principle of synthesis nanoparticles by microemulsion, the resultant particles are cubic fluorite structure with size of about 3 nm and well dispersed. Research results may approve that the system of CTAB/butanol/ cyclohexane/water solution could reduce the aggregation efficiently and obtain the nanoparticles with controlled size and narrow size distribution.