The effect of cation dopant on tetragonal to monoclinic (t-m) phase transformation of zirconia is examined, which causes the degradation of zirconia in vivo. The transformation behavior is much different depending of the dopant. The transformation is promoted when the pentavalent element is added. In contrast, the transformation is suppressed when the bivalent element is added. In case the tetravalent element is added, the transformation is promoted if the ionic radius of the element is larger than that of zirconium, and that is suppressed if the element with smaller ionic radius is added. The valence and the ionic radius of dopant must be important factors for the phase stability of zirconia.