In applications of yttria-stabilized zirconia (Y-TZP) ceramics as structural materials their thermal instability in the range 150-300 °C limits applications at high temperatures in spite of their superior mechanical properties (that is, high strength and fracture toughness, good wear resistance, etc.) [1-3]. From observations in [4-7], this thermal behaviour of Y-TZP degraded its mechanical properties and its so-called low-temperature degradation behaviours. Degradation behaviour during ageing is known to occur by an isothermal phase transformation from the tetragonal to the monoclinic phase which creates many cracks on the transformed surface. Also, this behaviour was found to be dependent on the grain size, the environment of water vapour, the ageing temperature and the content of the solute, etc. Several mechanisms of isothermal phase transformation in Y-TZP were proposed in [5-7], however these phenomena still have no clear explanation. In this present study, we investigated the reversed transformation (from the monoclinic to the tetragonal phase) by annealing at high temperatures and the related crack healing in aged 2Y-TZP specimens. The starting powder was TZ-2Y powder prepared by Tosoh Co. This powder contained a metastable tetragonal phase and a stabilizer of 2 mol % Y203. A cylindrical compact disk was prepared by cold isostatic pressing at a pressure of 200 MPa, and this was sintered at 1450 °C in air for a 20 h soaking time. The relative density of the sintered body was measured using Archimedes' method, and the grain size was measured from scanning electron microscopy (SEM) micrographs of the polished surface. Ageing of polished specimens was performed at 250 °C in a humid atmosphere for 20 h, and then annealing was performed in the temperature range 500-1200°C for aged specimens in an electric furnace. Soaking on annealing was performed for 1 h at each temperature. The amount of transformed phase accompanying ageing and annealing was analysed by X-ray diffraction (XRD) of the surface, and the ratio of the tetragonal to the monoclinic phase was determined by using the GarvieNicholson formula [8]. The microstructures of the annealed specimens were observed by SEM. On the polished surface of specimens sintered for 20 h, the tetragonal-phase content was about 90% and the monoclinic phase content was about 10%,