We have synthesized ceria nanoparticles by reacting cerium salts and sodium hydroxide without any additive in order to avoid possible cytotoxicity. The synthesized ceria nanoparticles were with cubic structure according to X-ray diffraction and have diameters between 6 and 10 nm analyzed by transmission electron microscopy. Aggregation behaviors of ceria nanoparticles in DI water and cell culture media have been studied by dynamic light scattering. In de-ionized water, ceria nanoparticles formed a stable colloidal solution with hydrodynamic size similar to 70 nm and zeta potential similar to 47 mV. Dispersion of the nanoparticles in serum-free media was found to result in larger aggregates, while the nanoparticles in serum-containing media form a stable solution, which suggests that proteins in serum and ceria nanoparticles have some interactions. In cellular uptake test, entry of ceria nanoparticles into HeLa cells incubated in serum-containing media at 37 degrees C for 60 min was observed and without cytotoxicity.