Studies on the formation of single-walled carbon nanotubes by an aerosol method based on the introduction of pre-formed catalyst particles into conditions leading to carbon nanotube synthesis are presented. Carbon monoxide and iron nanoparticles were used as carbon source and catalyst, respectively. The carbon nanotube growth was found to occur at a temperature of around 900 °C in the heating section of the reactor with a growth rate exceeding 1.1 μm/s. A detailed analysis of possible processes during carbon nanotube formation revealed heptagon transformation as a limiting stage. A mechanism for carbon nanotube formation on the basis of the geometrical consideration was proposed.