Nanostructured cobalt oxide (Co 3 O 4) has been regarded to be of great interest for catalysis, gas sensors, and energy conversion/storage. In this paper, we reported a water vapor-assisted thermal oxidation approach for the scalable synthesis of free-standing Co 3 O 4 nanowires with controlled size, crystallization, and spatial density. Thermodynamic calculation was carried out to understand the fundamental mechanism of the water vapor-assisted nanowire formation process. For the first time, we found the incorporation of water vapor in the growth system modified the nanowires formation pathway. Cobalt hydroxide (Co(OH) 2) was recognized to take a critical intermediate role during the growth. The proposed mechanism was further verified by understanding the influence of various growth parameters on the thermal oxidation process and product morphology. It was observed that the feeding rate of water vapor and type of oxygen source have a critical influence on the spatial density of Co 3 O 4 nanowires while the temperature shows a dominant effect on the product morphology. Finally, the representative Co 3 O 4 products were characterized for their composition, optical and magnetic properties. This study accumulates deeper understanding on the fundamentals of water vapor-assisted thermal growth of metal oxide nanostructures and successfully provides a facile approach for the scale production of Co 3 O 4 nanowires for future applications.