It is clear that a tetragonal phase transforms to the monoclinic and rhombohedral phases in grinding of ZrO 2 doped with 3 mol% Y 2 O 3 . The grinding process also changes the roughness of the surface. Because surface roughness and structural changes in the deformed surface layer should influence the mechanical properties of ZrO 2 -Y 2 O 3 , the bending strength σ b of ground specimens is presented as a function of both the volume of martensitic transformed phase and the roughness of the surface. Then, σ b is assumed to be equal to σ 0 (1 + ε m + r + γ), where σ 0 is the original bending strength of the sintered material polished to a surface roughness of 0.1 μm, and ε m + r and γare the degrees of influences caused by phase transformation and surface roughness. Furthermore, ε m + r and γ were defined respectively by the following equations of 1.71(V m - 1.3)100 and-0.55(R m a x - 0.1)100 , where V m is the volumetric percentage (vol%) of monoclinic phase, and R m a x is the roughness of the abraded surface. The bending strength increased as much as 2-14% because of phase transformation and decreased as much as 2-5% because of an increase in surface roughness. The degrees of influence of ε m and ε r by both phase transformations were approximately 6 and 5.8%.