Vertically-aligned single walled carbon nanotubes (V-SWNTs) were prepared by water-assisted and ethylene-pyrolyzing chemical vapor deposition. Water vapor was introduced into the growth environment by passing a small fraction of Ar carrier gas through a water bubbler. The effects of processing gas flow rate, growth time, and water vapor content on the growth of SWNTs using Mo/Fe as the catalyst were systematically studied. For the catalyst of SWNTs growth, multi-metal layers consisting of an Al underlayer with 10 nm in thickness and Mo/Fe layers on top were deposited onto the silica film. Evidences of V-SWNTs with high-purity were found by Raman spectroscopy and TEM morphology. The trace amount of water vapor enhanced the activity of metal catalysts to further assist the growth of SWNTs. For the Mo (0.5 nm)/Fe (1 nm) structure, V-SWNTs grew from surface when the growth time lasted 10 min shown in SEM diagram meanwhile the portion of SWNTs increased until 30 min and then a layer of SWNTs developed shown in the RBM peak in the Raman spectra.