We report the fabrication and electrical characteristics of scalable nanowall networkdevices and their gas sensor applications. For the network device fabrications,two-dimensional ZnO nanowall networks were grown on AlN/Si substrates with a patternedSiO2 mask layer using selective-area metal-organic vapor-phase epitaxy. The ZnO nanowalls withc-axis orientation were heteroepitaxially grown on AlN/Si substrates, and weresingle-crystalline, as determined by x-ray diffraction and transmission electron microscopy. Theelectrical conductivity of the nanowall networks was measured as a function of nanowalldimensions. The conductance increased linearly with the channel width for widths larger than1 µm, but saturatedat 36 µS forwidths below 1 µm. This conductance scaling behavior is explained by enhanced conduction throughthe regions near the edge of the patterned growth areas, where the density ofthe networks was higher. Gas sensor applications were investigated using thenanowall network devices, and highly sensitive gas detection was demonstrated.