Ultra-flat Zinc oxide (ZnO) films with natural hydrophobicity were sputtered onto glass substrates by glancing angle deposition (GLAD) method without addition of active oxygen at room temperature under different glancing angles relating to the sample holder. The sample holder was positioned at glancing angles of 0 degrees and 30 degrees, and the sputtering power was fixed at 75 W with low argon (Ar) pressure of 1 x 10(-2) Torr during deposition process. According to analysis of surface composition and structure, the naturally hydrophobic wetting state can be attributed to the different grain structure and hydrocarbon adsorbates on the top of the film surface. On the other hand, the interfacial water molecules near the surface of ultra-flat ZnO films are confirmed belong to the hydrophobic hydrogen structure by Fourier transform infrared/attenuated total reflection. In addition, the water contact angle was significantly improved by a simple factor of glancing angle. The water contact angle value of ultra-flat ZnO films increased from 90 degrees to 98 degrees while the sample holder is with glancing angle of 30 degrees. Moreover, our present ultra-flat ZnO films also exhibited excellent transparency over 80%, and the surface wetting switched from hydrophobic to hydrophilic states after exposing in ultraviolet (UV) surroundings. Then, the ZnO films could be freely and stably reversed back to hydrophobicity after stored in dark surroundings. This present study not only demonstrates that the natural wettability of ultra-flat ZnO films is strongly associated with surface composition and structure, but also provides an easy way to modulate and improve the surface wettability. This also extends the potential applications of ultra-flat ZnO thin films and aids a profound understanding for device design and material development. (C) 2017 Elsevier B.V. All rights reserved.