The effect of grain boundary segregated dopant on phase stability of tetragonal zirconia polycrystal (TZP) is examined by accelerated exposure tests ageing in hot water. The materials used in this study are 3 mol%Y2O3 stabilized TZP (3Y-TZP) and 0.1mol%SiO2-doped 3Y-TZP. Accelerated exposure tests in an autoclave reveal that the tetragonal phase stability of 3Y-TZP in water is highly affected by the grain boundary segregated dopant and the grain size. When the grain size of TZP is about 0.55μm, the change in phase transformation behavior with dopant is explained from the change in grain boundary diffusivity of hydroxyl ion. Grain boundary diffusion of hydroxyl ion must be blocked by the presence of some segregated ion which reduces the effective area of grain boundary diffusion. On the other hand, when the grain size is about 0.35μm, the phase transformation behavior seems to be controlled by the grain boundary stress. Decreased grain boundary stress by the segregation of some dopant must enhance the phase transformation of 3Y-TZP.