Effect of water vapor on the thermally induced crystallization of tetragonal ZrO2 in hydrous gel precursor was investigated using TG-DTA under controlled water vapor pressures, p(H2O). Existence of the temperature region for preliminary nucleation in the region lower than that of the crystallization exothermic peak was identified. The rate of preliminary nucleation was largely enhanced by the atmospheric water vapor, resulting in the increase of number of preexisting nuclei for the subsequent growth and the systematic shift of DTA exothermic peak of crystallization to the lower temperatures with increasing p(H2O). From a systematic shift of the DTA exothermic peak depending on the annealing time at a constant temperature, the rate behavior of preliminary nucleation under high p(H2O) was characterized as the constant rate nucleation. By the preannealing treatment in the temperature region of preliminary nucleation under high p(H2O), the number of preexisting nuclei is nearly saturated without any partial crystallization. From a series of DTA exothermic peaks of crystallization recorded by heating the preannealed samples at different heating rates, the apparent activation energy, Ea, for the crystal growth of tetragonal ZrO2 from the preexisting nuclei was estimated to be Ea = 187.3 ± 3.8 kJ mol−1.