Vertically aligned carbon nanotubes (CNT) were synthesized on Ni/Si substrates using microwave plasma-enhanced chemical vapor deposition, and the effects of nitrogen in the gas mixture of CH4+H-2+N-2 on the growth rate and the diameter of the nanotubes were studied. The growth rate and the diameter of CNT were systematically controlled by controlling the nitrogen content in the feed gas. With increasing the nitrogen content in the feed gas, the growth rate of the nanotubes increased, whereas the diameter decreased except for the case when nitrogen was not introduced. A model of roles of nitrogen in terms of etching carbon and Ni catalyst metal was suggested.