ZrO2 small particles with different crystal habits were prepared from zirconium nitrate under hydrothermal conditions. The ZrO2 particles were characterized by XRD, TEM and SEM. The crystal structure, morphology and size of the ZrO2 particles were strongly dependent on the glycerin additive in the reaction solution. The phase-pure t-ZrO2 with the size about 10 nm formed from the reaction solution added with glycerin while the rod-like phase-pure m-ZrO2 with the size about 30 x 80 nm formed from the reaction solution without glycerin (180 degrees C, 18 h, pH = 13). The t-ZrO2 could also form under the same hydrothermal conditions in the presence of other polyhydric alcohol additives, such as trimethylolpropane and tetramethylolmethane. But monohydric and dihydric alcohols were not benefits to formation of phase-pure t-ZrO2. The mechanisms for formation of phase-pure t-ZrO2 were supposed according to the experimental results.