The 40mol% CeO2-stabilized ZrO2 ceramic was synthesized by the sol-spray pyrolysis method and aged at 1400 degrees C-1600 degrees C. The effects of high-temperature aging on its fracture toughness were investigated after heat treatments at 1500 degrees C for 6-150h in air. Characterization results indicated that the activation energy for grain growth of 40mol% CeO2-stabilized ZrO2 was 593 +/- 47kJ/mol. The average grain size of this ceramic varied from 1.4 to 5.6m within the aging condition of 1500 degrees C for 6-150h. The Ce-lean tetragonal phase has a constant tetragonality (ratio of the c-axis to a-axis of the crystal lattice) of 1.0178 during the aging process. It was found that the fracture toughness of 40mol% CeO2-stabilized ZrO2 was determined to be 2.0 +/- 0.1MPam(1/2), which did not vary significantly with prolonging aging time. Since no monoclinic zirconia was detected in the regions around the indentation crack-middle and crack-tip, the high fracture toughness maintained after high-temperature aging can be attributed to the remarkable stability of the tetragonal phase in 40mol% CeO2-stabilized ZrO2 composition.