Abstract: The present invention relates to a process for preparing single wall carbon nanotubes (SWCNT) having a diameter dSWCNT, which comprises (i) providing a precursor element which comprises a segment SSWCNT of the single wall carbon nanotube, the segment SSWCNT being made of at least one ring formed by ortho-fused benzene rings, and having a first end E1 which is open and a second end E2 which is opposite to the first end E1, (ii) growing the precursor element by vapour phase reaction with a carbon-source compound on the surface of a metal-containing catalyst, wherein the precursor element is in contact with the surface of the metal-containing catalyst via the open end E1 of the segment SSWCNT, and the metal-containing catalyst is in the form of particles having an average diameter dcat satisfying the following relation: dcat>2×dSWCNT or in the form of a continuous film.