Tetragonal to monoclinic phase transformation of yttria-stabilized zirconia, namely plasma-sprayed coatings and sintered bodies containing 4-8 mass % Y2O3 during hydrothermal aging was investigated with respect to Y2O3 distribution using 1 mu m phi area from electron probe microanalysis (EPMA) and 20 nm phi area from transmission electron microscopy (TEM) analysis. Phase transformation at 473 K was prevented only in plasma-sprayed coatings having more than 6.7 mass % Y2O3 in 20 nm phi microscopic area. Furthermore, it was confirmed influence of Y2O3 distribution on the rate constants of this phase transformation was observed at 368 K.