Aligned carbon nanotubes were grown by pyrolysis of ferrocene and acetylene in the temperature range 700-1000degreesC. The average diameter is constantly 20 run for all growth temperatures. As the temperature increases, the growth rate increases by 60 times. The length reaches up to 3 mm at 1000degreesC. These long carbon nanotubes exhibit a cylindrical structure. The relative amount of crystalline graphitic sheets increases significantly with the growth temperature. The Arrhenius plot yields the activation energy 35 +/- 3 kcal/mol, which is close to the diffusion energy of carbon in bulk gamma-Fe. We suggest that the bulk diffusion of carbons would play an important role in the growth of cylindrical structured carbon nanotubes. (C) 2003 Elsevier Science B.V. All rights reserved.