The deformation behavior of nanocrystalline monoclinic ZrO2 polycrystals (nanocrystalline MZP) was studied at 1273-1373 K in compression tests. The deformation of nanocrystalline MZP was characterized by stress exponent (n = 2.5), grain-size exponent (p = 2.5), and apparent activation energy (Q = 350 kJ/mol). The values of n and p were similar to the superplasticity of high-purity Zn-22% aluminum alloy. The strain rate of nanocrystalline MZP was faster than that of Y2O3-stabilized tetragonal ZrO2 (Y-TZP) at temperatures lower than the monoclinic-tetragonal transition temperature. The strain rate of MZP gradually approached to that of Y-TZP as the temperature increased to the transition temperature. The comparison of present data with published data suggested that trace amount of impurities affected the deformation behavior of MZP.