Vertically aligned carbon nanotubes (VA-CNTs) were successfully grown on silicon substrates by atmospheric pressure chemical vapor deposition at 750∘C using acetylene as a carbon source and Fe-Co nanoparticle thin films as a cocatalyst. Preparation of the cocatalyst was operated by an economical sparking process. A small amount of water vapor was introduced into the reactor by controlling diffusion by heated water in a flask. The CNTs were characterized by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The VA-CNTs were obtained only when the water vapor fraction in the introduced gas was in the range of 310–440 ppm. The length of the VA-CNTs reached 0.8 mm with a growth rate of 17 μm/min. Moreover, D/G-band ratio suggests that the optimum fraction of water vapor decreases defects in CNTs.