The ultrafine ZnO nanoparticles/nanowires were successfully synthesized on a flexible and transparent substrate by an ultraviolet-light decomposition process. We demonstrate that water molecules can affect the morphology of ZnO nanostructures. An ultraviolet lamp (λ ~ 380 nm, 75 mW cm(-2)) can be used to irradiate Zn(AcAc)2 and Zn(AcAc)2·H2O precursors, which rapidly synthesize ZnO nanoparticles and nanowires, respectively. High-resolution transmission electron microscopy (HRTEM) images and a selected-area electron diffraction pattern revealed that the single-crystal nanoparticles were comprised of wurtzite structure ZnO. The nanowires consisted of ultrafine nanoparticles. On the basis of the Debye-Scherrer formula, the particle size of ZnO was calculated as ~6-9 nm. The more water molecules the precursor had, the more OH(-) and Zn[(OH)4](2-) it put out. Moreover, due to the Zn[(OH)4](2-) and Zn(OH)2 species formed on the surface of the ZnO nanocrystals, they facilitated the one-dimensional nanowires during the crystal growth process. On the basis of our investigations, oxygen vacancies, hydroxyl, and zinc hydroxide all acted as key components in the formation processes that determined photoresponsive properties.