Low-crystalline pyrochlore powder of lead magnesium niobate was treated under various hydrothermal conditions. The perovskite phase crystallized in pure water above 580 degrees C, but hydrothermal treatments at high temperatures for a long time decreased the amount of the perovskite phase and increased that of the cubic pyrochlore phase. The addition of lead oxide enhanced the formation of the perovskite, but not magnesium oxide. The perovskite phase was not stable in many salt solutions even with the addition of lead oxide, but its stability was increased in solutions consisting of large anions and cations.