Nanosized powders of ZrO2-CaO solid solutions have been synthesized by hydrothermal treatment at 110 degrees C. Amorphous hydrous ZrO2, CaO freshly prepared by thermal decomposition of CaCO3 and MOH (M = Na, Li) mineralizer solutions were employed as precursors for the hydrothermal synthesis. Calcia-stabilized zirconia (CSZ) with different CaO content and characterized by different crystal sizes have been produced by changing the hydrothermal treatment temperature, the reaction time and the mixture composition of precursors. The combined effect of both MOH and CaO on the crystallization-stabilization of zirconia at low temperature of hydrothermal treatment is discussed.