Grain-size-dependent superplastic deformation (SPD) is examined in terms of the cooperative grain-boundary sliding (CGBS) of 3mol% yttria-partially stabilized tetragonal zirconia polycrystals (3Y-TZP) with various grain sizes ranging from 0.30mum to 0.65mum. The true stress and true strain curves (S-S curves) in tensile tests at 1300degreesC exhibit a significant size effect of sintered grains. The relationship between the maximum flow stress sigma(max) in S-S curve and the mean radius R of grains is represented by sigma(max) proportional to R. Theoretical considerations are made on the basis of the present test results.