Transmission electron microscopic study on the aligned carbon nanotubes has demonstrated a growth mechanism which involves two sizes of iron nanoparticles. While the small particle is catalytically active for the nucleation of the nanotube, the large particle produces the carbon atomistic species required for the growth of the nanotubes. The aligned nanotubes are believed to be the result of a competition growth process along the normal direction of the substrate. The surface diffusion of carbon atoms on the large iron particle leads to the formation of the observed bamboo-like structure.