This paper focuses on the growth mechanism of the multiwalled carbon nanotubes exhibiting helical and zigzag growth morphologies. Our data support a kinetically controlled growth model in which the creation rates of the pentagon and heptagon carbon rings determine the geometrical shapes of the nanotubes. The carbon nanotube is believed to be grown from a carbon cluster that is nucleated from a pentagon carbon ring followed by a spiral shell growth. The pairing of pentagonal-heptagonal (P-H) carbon rings is essential in forming the helical and zigzag structures. If there is no twist in the P-H orientation along the body of the carbon nanotube, a planar-spiral structure would be formed. If the P-H pairs twist their orientations along the growth direction of the nanotube, a helical structure would be formed. The periodicity and the coiling diameter of the helix are determined by the angle of the twist and the distance between the adjacent P-H pairs. If the P-H pairs are densely accumulated at a local region and the interpair distance is small, a node is formed, thus, the zigzag growth morphology.