Precursors of CeO2 were synthesized in a reverse microemulsion composed of cetyltrimethylammonium bromide (CTAB), 1-butanol, 1-octane, Ce(NO3)3 brine (ammonia). The optimum calcination temperature (550 ℃) was derived from thermogravimetry (TG) analysis and X-ray powder diffraction (XRD). The CeO2 nanoparticles were then prepared by calcining the precursors at 550 ℃. The structures, morphologies, size, and UV absorption properties of the CeO2 nanoparticles were characterized by XRD, transmission electron microscopy (TEM), and UV-Vis spectroscopy. Cubic crystalline CeO2 nanoparticles of 5 -18 nm in size and with od monodispersity were obtained using this method. The influences of the mass ratio of 1-octane to 1-butanol and the concentration of Ce(NO3)3 on the size of the CeO2 nanoparticles were studied. Results showed that CeO2 nanoparticle size was dependent on the mass ratio of 1-octane to 1-butanol and the concentration of Ce(NO3)3. The size of the CeO2 nanoparticles decreased as the mass ratio of 1-octane to 1-butanol as well as the concentration of Ce(NO3)3 increased. In addition, we studied the UV absorption properties of the CeO2 nanoparticles by UV-Vis spectroscopy.