We report the synthesis of faceted single crystalline ZnO nanonecklace (ZnO NN) arrays horizontally aligned on r-plane sapphire using Au nanoparticles catalyzed chemical vapor deposition. High resolution TEM data show that ZnO NNs, without any grain boundary observed, grow along the ZnO [0001] direction horizontally on r-plane sapphire and also reveal the epitaxial relationships between the ZnO NN and r-plane sapphire with ZnO [0001] ∥ sapphire [101̅1] and ZnO (1̅21̅0) ∥ sapphire (011̅2). It was found that the Au nanoparticles with diameter of ∼5 nm and lower particle density are critical for the formation of horizontally aligned ZnO NN arrays, while the larger size or the higher density of Au nanoparticles results in other types of ZnO nanostructures, such as vertical ZnO nanoblades and nanowires. Thermodynamic analysis indicates that faster increase of strain energy compared to slower increase of surface free energy and interfacial energy with size increase of one-dimensional ZnO nanostructures might be the critical reason for the size effect that controls the vertical vs horizontal growth of ZnO nanostructures. The smaller lattice mismatch of 1.5% in the growth direction (ZnO [0001] ∥ sapphire [101̅1]) compared to the larger lattice mismatch of 18.3% in the other horizontal direction (ZnO [101̅0] ∥ sapphire [21̅1̅0]) that is normal to the growth direction contributes to the one-dimensional growth. The width of ZnO NNs increases with growth time, indicative of continuous postgrowth deposition of ZnO on ZnO NNs. The evolution of necklacelike structures of ZnO might be related to liquid catalyst surface tension, lattice mismatch/strain energy, and surface decoration of ZnO facets with Al and Au atoms.