Pure zirconia and $x$ mol% calcia partially stabilized zirconia ($x$ = 1.5, 3, and 8) nanopowders were synthesized by hydrothermal method with various reaction temperatures for 24 hrs. The precipitated precursor of pure zirconia and $x$ mol% calcia doped zirconia was prepared by adding $NH_4OH$ to starting solutions; resulting sample was then put into an autoclave reactor. The optimal experimental conditions, such as reaction temperatures and times and amounts of stabilizer CaO, were carefully studied. The synthesized $ZrO_2$ and $x$ mol% CaO-$ZrO_2$ ($x$ = 1.5, 3, and 8) powders were characterized by XRD, SEM, TG-DTA, and Raman spectroscopy. When the hydrothermal temperature was as low as $160^{circ}C$, pure $ZrO_2$ and $x$ mol% CaO-$ZrO_2$ ($x$ = 1.5 and 3) powders were identified as a mixture of monoclinic and tetragonal phases. However, a stable tetragonal phase of zirconia was observed in the 8 mol% calcia doped zirconia nanopowder at hydrothermal temperature above $160^{circ}C$. To observe the phase transition, the 3 mol% CaO-$ZrO_2$ and 8 mol% CaO-$ZrO_2$ nanopowders were heat treated from 600 to $1000^{circ}C$ for 2h. The 3 mol% CaO-$ZrO_2$ heat treated at above $1000^{circ}C$ was found to undergo a complete phase transition from mixture phase to monoclinic phase. However, the 8 mol% calcia doped zirconia appeared in the stable tetragonal phase after heat treatment. The result of this study therefore should be considered as the preparation of 8 mol% CaO-$ZrO_2$ nanopowders via the hydrothermal method.