The design of the 1D nano-structured material arrays on substrates is required for the development of conductivity sensors. A simple method, the so-called gas-solution-solid, has been developed to prepare ZnO nanorod arrays on Zn substrate. The method consists of putting Zn foils above an ammonia solution for about 24 h to obtain the aligned ZnO nanorods on substrates. The sizes of ZnO nanorod arrays could be controlled by adjusting the concentration of the ammonia-water solution and reaction time. Moreover, the growth mechanism of the nanorod arrays was studied and proposed. In the proposed mechanism, the Zn source from the Zn foil is first oxidized to ZnO and Zn2+ by oxygen. Then ZnO nanorod arrays are grown with the help of ammonia in the solution as a transporter of zinc ions. The nanorod arrays growth mechanism involves oxygen gas, ammonia solution, and Zn foil, and is, therefore, named as gas-solution-solid mechanism. Finally, the NH3 sensing properties were measured and the results showed that ZnO nano-array sensor has high sensitivity and reversibility for NH3 sensing at the room temperature and the sensitivity increases with increasing gas concentration.