The effect of small amount of cation dopant on phase stability of zirconia bioceramics is evaluated by ageing in hot water. The phase transformation progresses with time in all specimens. However, the tetragonal to monoclinic transformation behavior is much different depending on the dopant. In case that the small amount of tetravalent soluble 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, which is explained from the change in the axial ratio, c/a. However, the change in phase stability of 7.7mol%TiO2-doped TZP cannot be simply explained from the change in the axial ratio, c/a. XRD analysis reveals that the distance between nearest neighbor anion and cation site significantly decreases only in 7.7mol%TiO2-doped TZP. This result indicates that the binding energy between dopant and oxygen vacancy affects the phase stability as well as the change in the axial ratio, c/a.