The ZnO nanoparticles were successfully grown from the nano-ZnO embedding in P(MMA-co-BA) matrix using simple hydrothermal treatment in ammonium hydroxide and zinc nitratehexahydrate precursor solution with varying pH and time. The newly-formed ZnO nanoparticles well adhered on the nano-ZnO/P(MMA-co-BA) composites because they grew from the nano-ZnO seeds in the P(MMA-co-BA) matrix. The pH and time of hydrothermal treatments affected not only on the morphology of newly formed ZnO nanostructures but also their amount and particle sizes. The higher amount of Zn(OH)4 2−nuclei would be created when the higher pH and the longer time of hydrothermal treatments were used, resulting in the multi-direction growth of ZnO nanostructures. The newly formed ZnO nanostructures increased the roughness of nanocomposite surfaces, inducing the increase of hydrophobicity. In addition, the hydrothermal formation of ZnO nanostructure could enhance the thermal stability of the nano-ZnO/P(MMA-co-BA) nanocomposite