Zinc oxide (ZnO) is an important multifunctional material with versatile applications in gas and biological sensors, solar cells, catalysts, electronics, piezoelectric devices and high-efficiency short-wavelength optoelectronic nanodevices owing to their wide bandgap (3.37 eV). Gas sensors based on ZnO nanostructures can be used to detect the concentration of ethanol vapor, carbon monoxide and ozone gas. Especially, due to their high surface-to-volume ratio, ZnO nanorods are commonly introduced into gas sensors in many applications. In this work, we report on hydrothermal growth of well-aligned ZnO nanorod arrays perpendicularly oriented on ZnO seed layers which were prepared on the substrate by atomic layer deposition (ALD). The ZnO nanorods were growth by hydrothermal method with zinc nitrate hexahydrate, hexamethylenetetramine, and DI water. The very fine grained ZnO seed layers were deposited by ALD and dense ZnO nanorod arrays were grown by hydrothermal process. The crystal structure of ZnO was analyzed by X-ray diffraction (XRD). ZnO nanorods morphology was inspected using a field emission scanning electron microscopy (FE-SEM) and ZnO seed layer was investigated by atomic force microscopy (AFM). The sensing performance of ZnO nanorod samples were investigated using a sensor testing system.