The effect of oxygen in the synthesis of oxide nanowires by using carbothermal reduction process have been studied thermodynamically and kinetically. By using laboratory air, ZnO nanowires could be fabricated in the carbothermal reduction process and a metal oxidation process. As the processing pressure decreases, the diameter of the nanowires decreases and the oxygen vacancy increases. As the processing pressure increases, the oxygen vacancy decreases and the shape of the ZnO becomes plate-like.