It has been well known that vertically aligned carbon nanotubes (CNTs) can be grown by thermal CVD in NH, environment. However, the mechanism of the vertically aligned CNT growth is yet to be clarified. In the present work, we investigated the effect of NH3 gas on the CNT growth in the viewpoint of catalyst passivation. The particles of Ni or Co of diameter ranging from 17 to 58 nm were used as the catalyst. CNTs were deposited at 950 degreesC using C2H2 in various environments of NH3, H-2, or their mixtures. In H-2 environment, significant catalyst passivation was observed at the CM, concentration of 2.4 vol.% due to the excessive supply of carbon. However, vertically aligned CNTs were deposited in NH, environment even when the C2H2 concentration was 16.7 vol.%. From the composition analysis of the catalyst surface, we could show that activated nitrogen atoms were generated by the decomposition of NH3. Two possible roles of the nitrogen were suggested based on the bamboo growth model. The nitrogen atoms enhance the formation of graphitic layer and/or improve the separation kinetics of the graphitic layer from the catalyst. The growth behaviors without the pre-treatment for 1 h in NH3 environment showed that the role of the nitrogen appeared in different ways depending on the catalyst materials. In the case of Co catalyst, where vertically aligned CNT growth was observed without the pre-treatment, enhanced formation of the graphitic layer might be the significant role of the nitrogen. However, when using Ni catalyst, the pre-treatment in NH3 environment was required for the CNT growth, which implies that both the formation and the separation of the graphitic layer were essential. 2001 Elsevier Science B.V All rights reserved.