A novel method based on utilizing varied ionic strength and specific adsorption was developed to prepare c-oriented zinc oxide (ZnO) films with controlled morphology in aqueous solution. Structure and morphology of the films were investigated by using X-ray diffraction, scanning electron microscopy, field emission scanning electron microscopy, and high-resolution transmission electron microscopy methods. A layer-by-layer growth manner of ZnO rod crystals was observed. The diameter of the ZnO rods can be controlled through the incorporation of the cost-effective strong electrolyte NaCl. The reason for the variation of ZnO-film morphology is that the enhanced ionic strength because of the addition of NaCl promotes not only the related reactants' concentration but also the specific adsorption of organic groups. Furthermore, acetyleacetone used as an assistant can play an important role in specific adsorption, leading to the formation of pyramid crystals through mild secondary growth.