Significant amounts of ceria nanoparticles (CeO2 NPs) are released into the environment, whereas their effects on living organisms are not clearly established. We investigated the long-term toxicity of CeO2 NPs to Balb/c Mice. Animals were exposed to 0, 100, 200, 400 and 800 mg/kg BW by intragastric administration (IGA) for 9 weeks. Weight was weekly recorded, whereas superoxide dismutase (SOD) activity and the levels of reduced glutathione (GSH) and malondialdehyde (MDA) were measured in livers and kidneys after exposure period. Although the weight of mice was not significantly affected, we observed a depletion of SOD and GSH contents at 400 and 800 mg/kg BW in the liver and kidney. Conversely, we noticed a substantial increase in MDA level in the liver at 400 and 800 mg/kg BW and in the kidney at 800 mg/kg BW. The results suggested that long-term exposure to CeO2 NPs induces oxidative stress in mice, and hence highlighted the toxicity of these nanoparticles to mammalians.