The effect of cation ion dopant on phase transformation of zirconia bioceramics is evaluated by ageing in hot water. The phase transformation progresses with time in all specimens. However, the transformation behavior is much different depending on the dopant. The transformation is promoted when the 1 mol% of pentavalent element is added to 3mol% yttria stabilized zirconia (3Y-TZP). In contrast, the transformation is suppressed when the bivalent element is added. In case that the tetravalent element is added, the transformation is promoted if the ionic radius of the element is larger than that of zirconium, and the transformation is suppressed if the element with smaller ionic radius than zirconium ion is added. This result indicates that the ionic radius and the valency of ions are important factors to control the phase stability in zirconia.