A simple, rapid and green chemistry approach to controlling the shape of ceria nanocrystals by tuning the interaction of organic molecules with various crystallographic planes of fluorite cubic ceria, using supercritical water as a reaction medium was investigated. Crystal structure and morphology of the synthesized ceria nanocrystals were characterized by means of powder X-ray diffraction, TEM, and HRTEM. The XRD patterns of the crystals indicated that they had a fluorite cubic structure. TEM images of the ceria nanocrystals with uncapped surfaces showed spherelike shapes with an average diameter of 7 nm, and the nanocrystals were aggregated. It was observed that these nanocrystals can be self-assembled into close-packed superlattices on carbon substrates. They can find application in a variety of areas, including photonics and catalysis.