Partially stabilized zirconia ceramics were sintered using fine powder of ZrO2-3mol%Y2O3 prepared by the chemical co-precipitation method.The tetragonal-to-monoclinic phase transformation in ZrO2 ceramics during the aging in boiling water and the effect of an applied stress of 100MPa were mainly investigated.The degradation of ZrO2 ceramics is considered to be caused by the reaction between Y2O3 and H2O,which leads to a decreasing in the stability of tetragonal phase of ZrO2.It is found that the tensile stress improves the driving force of the phase transformation and accelerates the degradation while the compressive stress has no obvious effect on the degradation.