The catalyst-free growth of ZnO nanostructures on Si and SiO2/Si substrates as a function of substrate temperature was carried out using a thermal evaporation method. We observed that the shapes and the morphologies of the ZnO nanostructures could be controlled by using the substrate temperature and the presence of an oxide layer on the surface of the substrate. The shape of the ZnO nanostructure was changed from an embossed nanocantilever to a nanowire as the growth temperature was decreased from 500 °C to 430 °C. At 360 °C, a winding stem-like nanostructure with thin and short branch nanowires on the facet of the nanostructure was produced. In particular, at a growth temperature of 430 °C, a ZnO buffer layer was formed during the initial growth when an Si substrate was used. However, no ZnO buffer layer was observed when a SiO2/Si substrate was used. The formation of a buffer layer significantly affected the crystalline structure. Defects were observed in the embossed nanocantilevers and the nanowires grown on SiO2/Si but not in those grown on Si.