The deformation behavior of nanocrystalline monoclinic ZrO2 (nano MZP) and Y2O3 stabilized tetragonal ZrO2 (nano Y-TZP) were studied at 1323 K in compression tests. Both nano Y-TZP and nano MZP were "high-purity" materials in which impurity content (such as Al2O3, SiO2 and Fe2O3) was less than 0.1 wt %. The stress exponent of nano Y-TZP was 2.7 at a high stress region, and increased with decreasing stress. The deformation of nano MZP was characterized by a stress exponent of 2.5 over a wide stress range. This suggests that the transition of stress exponent in the deformation of high-purity Y-TZP is related to the doped yttrium.