The reduction treatment effects on the flexural strength and the fracture toughness of ceria were evaluated at room temperature. The results reveal that the flexural strength decreases significantly after reduction under very low oxygen partial pressures; however, in contrast, fracture toughness is increased by 30%-40% when the oxygen partial pressure was decreased to 10(-20)-10(-22) atm range. The large microcracks and the surface tensile stresses developed upon reduction treatment and cooling process are responsible for the decreased strength. The enhancement in toughness is also discussed in terms of internal stresses, microcrack toughening, and crack deflection mechanisms.