ZnO nanostructures are grown on Si and GaAs substrates by chemical bath deposition (CBD) method. Scanning electron microscope (SEM) measurements indicate that ZnO nanorods are formed on Si substrate, in contrast to the nanorods, network ZnO nanowalls are obtained on GaAs substrate. To elucidate the mechanism of ZnO nanostructures grown by CBD method, the morphology evolution mechanisms of the ZnO with growth time and solution concentration nanostructures are investigated systematically. A corresponding growth model from the view of crystal nucleation and growth process is proposed. The understanding of morphology evolution of ZnO nanostructures on different substrates will facilitate the controllable growth of ZnO nanostructures and therefore crucial for the application of ZnO nanodevices.