Highly crystalline and dispersible zirconia nanoparticles, ex situ synthesized from a solvothermal reaction of zirconium(IV) isopropoxide isopropanol complex in benzyl alcohol, were functionalized with 3-(trimethoxysilyl)propyl methacrylate and blended with UV-curable urethane-acrylate formulations to fabricate poly(urethane-acrylate)/zirconia (PUA/ZrO2) nanocomposite coatings. A critical ZrO2 concentration of 20 wt% was observed for the evolutions of both the structure and properties of the nanocomposites as a function of ZrO2 content. Below the critical concentration, completely transparent nanocomposite film was obtained and the nanocomposites exhibited increasing final carbon-carbon double bond conversion, refractive index, hardness, elastic modulus, and thermal stability as ZrO2 content increased. However, serious agglomeration of ZrO2 nanoparticles occurred at 25 wt% of ZrO2, which decreased final conversion, transparency and hardness, and thermal stability of the nanocomposite film. These results clearly reveal that the performance of UV-curable nanocomposites is strongly dependent on the dispersion of nanoparticles.