[en] We modulated the shape of ZnO nanostructures as nanowires, nanospears, nanowalls, and nanorods by varying the distance between the reactant and the substrate during thermal chemical vapor deposition. Photoluminescence (PL) data show that the optical quality measured by using the PL intensity ratio of band-edge emission to yellow emission is relatively higher under the synthesis condition of nanowalls. The formation characteristics of the ZnO nanostructures are explained by using an analytical model. The model entails a change in the growth mechanism of the ZnO nanostructures caused by a difference in the Zn concentration.