We report a synthesis approach using Si-nanocrystal (NC) layer that is formed by annealing Si-rich oxide with different oxygen content (stoichiometry, x) for growing vertically-aligned multiwalled carbon nanotubes (VA-MWCNTs). VA-MWCNTs with largest length and diameter of about 190 mm and 20 nm, respectively, were grown at x = 1.6. The atomic-resolved transmission electron microscopy image of the tube wall at x = 1.6 revealed almost-straight and well-separated graphitic sheets without defects, possibly resulting from the highest-quality Si NCs at x = 1.6. Active iron catalyst particles were formed on the Si-NC layer resulting in the formation of highly-aligned MWCNTs. Possible mechanisms are described to explain the experimental results.