The effect of second phase particles on tetragonal to monoclinic (t-m) phase transformation of zirconia is examined, which causes the degradation of zirconia in vivo. Silica is selected as the second phase particles and dispersed 0.1 to 10mol% into 3mol% yttria stabilized tetragonal zirconia polycrystal (3Y-TZP). As the result, phase transformation is promoted in all the samples examined in comparison with 3Y-TZP. Non-dispersed specimen exhibits better phase stability than that in silica doped one. Since the thermal expansion coefficient is smaller in order of silica and zirconia, the residual stresses induced during cooling from the sintering temperature must be the dominant factor to determine the phase stability in these materials.