Different carbon nanotubes are formed by substrate free metalorganic chemical vapor deposition of Fe catalysts and C2H4. A strong supersaturation of the Fe precursor in the gas phase was seen in the beginning of the deposition zone. The Fe particles was expected to be decomposed through an enhanced gas phase nucleation, leading to the formation of nanometric Fe particles. The C2H4 would react with the Fe particles in the gas phase following catalytic process, yielding different C products. The density and size of Fe particles, the C/Fe ratio, the operating temperature and the duration of the exposure of the reactants in the deposition zone would favor the formation of nanotubes to the expense of other forms of carbon such as soots and pyrolitic C.