Ion product scale for phase and size selective nanoparticle growth of tetragonal and monoclinic zirconia was determined from a series of in situ synchrotron radiation X-ray powder diffraction of hydrothermal synthesis with 13 different temperature and pressure conditions including nearly supercritical and supercritical water synthesis. The precursors of each synthesis were completely identical in the present study, and only properties of water by changing temperature and pressure were different in each condition. The crystalline phases and sizes of the particle were characterized by the Rietveld analysis of powder diffraction data. The conditions for the growth of tetragonal and monoclinic crystals were found in the series of results. The conditions were able to be organized by the ion product of the water. The growth of tetragonal crystals was observed in the ion product range from 10^–11.8 to 10^–14. The growth of monoclinic phase was observed in the ion product larger than 10^–11.8. No crystal growth was observed in the ion product smaller than 10^–18. The sizes of synthesized zirconia nanocrystals in the 10 kinds of experimental conditions were classified by 3 kinds, 20, 25, and 12 nm, for tetragonal and 2 kinds, 8 and 10 nm, for monoclinic phases. The sizes can also be organized by the ion product of water.