Ceria nanostructure-based catalysts have attracted much attention in recent years because of their unique physiochemical properties. Herein, we have presented a simple two-phase approach for the synthesis of ceria nanocrystals. Structural and morphological characterization by XRD and TEM showed that the as-synthesized monodisperse CeO2 nanoparticles (NPs) had cubic fluorite crystal structure with average particle size about 6.75 nm. The effects of hydrothermal temperature, annealing time, and concentration of cerium nitrate on the nanostructures of the products were also investigated and discussed. In addition, the CeO2 nanocrystals proved to be an effective catalyst for the photodegradation of blue methylene under UV illumination.