The controllable synthesis of metal oxide nanoparticles is of fundamental and technological interest. In this article, highly transparent aqueous nanodispersion of ZrO2 with controllable crystalline phase, high concentration, and long-term stability was facilely prepared without any modification via the reaction of inexpensive inorganic zirconium salt and sodium hydroxide in water under an acid surrounding, combined with hydrothermal treatment. The as prepared transparent nanodispersion had an average particle size of 7 nm, a high stability of 18 months, and a high solid content of 35 wt %. ZrO2 nanocrystals could be readily dispersed in many solvents with high polarity including ethanol, dimethyl sulfoxide, acetic acid, ethylene glycol, and N,N-dimethylformamide, forming stable transparent nanodispersions. Furthermore, highly transparent polyvinyl alcohol/ZrO2 nanocomposite films with high refractive index were successfully prepared with a simple solution mixing route. The refractive index could be tuned from 1.528 to 1.754 (@ 589 nm) by changing the mass fraction (0-80 wt %) of ZrO2 in transparent nanocomposite films.