The relationship between the starting temperature of the martensitic transformation, M(s), and the grain size of the parent phase, d, in ZrO2-containing ceramics was investigated. The experimental results showed that in tetragonal zirconia polycrystals doped with CeO2 (8 mol%) and Y2O3 (0.25 mol%) (8Ce, 0.25Y-TZP), the M(s) temperature displays a linear relationship with d(-1/2), its slope being negative. A new explanation for this phenomenon, the so-called the size effect, has been presented, in which the grain size of the parent phase affects the M(s) temperature through the strength of the parent phase. Thermodynamic calculation of the relationship between M(s) and d gives a result consistent with the experimental ones.