Single crystalline ZnO nanowires were synthesized by hydrothermal process and then formed nano-tubes by acidic etching these nanowires in acetic solution at 85 degrees C. The nanotube diameter can be easily controlled by dividing the nanowires growth and etching process. The ZnO nanotubes remain single crystalline hexagonal structure after the etching process. The defects existed in the nanowires and the dangling bonds of the nanowires' surface play the important roles for the etching process. An etching model for forming ZnO nanotubes is proposed, which can be proved by our experimental results.