Room temperature stability of metastable tetragonal-zirconia microcrystals in their composite ceramics has been argued from the standpoint of the constraint force (σ) of matrix, which was calculated by Eshelby's equation. The equation is based on the assumption that the dispersed particles and matrix are independent each other and then the effect of the interface can be ignored. However it was found that there was no significant relations between the critical size (d_c) (an index of stability) of the microcrystals and the constraint force. This means that the interaction (the effect of the interface) between the dispersed particles and the matrix cannot be ignored in these materials. The t→m phase transformation of ZrO₂ was influenced by not only “matrix effect” and “surface energy one” but also “interface one”. It was insisted that a kinetical discussion was also important in analysing the transformation mechanism. It seems to be probable that the field strength (Z/a₂) of the metal ions existing in the interfacial layer effects on the d_c value.