We have succeeded in the simple and rapid synthesis of the hydrophilic polymer-modified CeO2 nanoparticles using a supercritical hydrothermal method. To prepare the nanoparticles, Ce(OH)4 as precursor was treated in a batch-type reactor with supercritical water in the presence of either polyvinyl alcohol (PVA) or polyacryl acid (PAA) as surface modifiers. The hydrophilic polymers attached to the surface of the CeO 2 nanoparticles by the coordination bond between the functional groups, such as hydroxyl (-OH) or carboxyl (-COOH), of the polymers and the Ce atoms. The amount of the attached polymers on the surface of the CeO2 nanoparticles tended to increase with a decrease in the molecular weight of the polymer. The morphology and the particle size of the nanoparticles were cuboctahedral and about 20 nm, respectively. The nanoparticles were dispersed in water by virtue of the functional groups on the polymers. Notably, the ζ potential of PAA-modified CeO2 nanoparticles did not become zero in the measured pH range between 3 and 11. Interestingly, the surface modification by the polymers controlled the band gap of the nanoparticles, suggesting the possibility of tuning the electronic and the optical properties of the metal oxide nanoparticle by modifying their surface with organic molecules.