The phase transformation of less than 6 mol% Sc2O3-doped tetragonal zirconia polycrystals consisting of fine grains has been investigated. Dense bodies with homogeneous microstructures doped with 3.5 to 5 mol% Sc2O3 sintered at 1300 degrees C for 1 h consisted mostly of a tetragonal phase. Within 20 h of aging under hydrothermal conditions at 180 degrees C, the amount of monoclinic ZrO2 on the surface of the 4 to 5 mol % Sc2O3-doped specimens sintered at 1400 degrees C was saturated and reached a constant value, and the increase in the amount of monoclinic ZrO2 showed a sigmoidal type of kinetic transformation. The apparent activation energy for the phase transformation in Sc2O3-doped zirconia was 84-91 kJ/mol. Based on the hydrothermal aging results, the possible existence of a larger two-phase (cubic+tetragonal) region is suggested, and the phase boundary between the cubic+tetragonal and cubic phase in the ZrO2-Sc2O3 system is proposed.