Three different ZnO nanostructures, dense nanorods, dense nanowires, and sparsenanowires, were synthesized between Pt electrodes by on-chip hydrothermal growth at90 °C and below. The three nanostructures were characterized by scanning electronmicroscopy and X-ray diffraction to identify their morphologies and crystal structures.The three ZnO nanostructures were confirmed to have the same crystal type, but theirdimensions and densities differed. The NO2 gas-sensing performance of the threeZnO nanostructures was investigated at different operation temperatures. ZnOnanorods had the lowest response to NO2 along with the longest response/recoverytime, whereas sparse ZnO nanowires had the highest response to NO2 and theshortest response/recovery time. Sparse ZnO nanowires also performed best at 300°C and still work well and fast at 200 °C. The current-voltage curves of the three ZnOnanostructures were obtained at various temperatures, and results clearly showed thatsparse ZnO nanowires did not have the linear characteristics of the others. Analysis ofthis phenomenon in connection with the highly sensitive behavior of sparse ZnOnanowires is also presented.