Method for preparing carbon nanotubes or nitrogen-doped carbon nanotubes by pyrolysis, in a reaction chamber, of a liquid containing at least one liquid hydrocarbon precursor of carbon or at least one liquid compound precursor of carbon and nitrogen consisting of carbon atoms, nitrogen atoms and optionally hydrogen atoms and/or atoms of other chemical elements such as oxygen, and optionally at least one metal compound precursor of a catalyst metal, in which said liquid is formed under pressure into finely divided liquid particles such as droplets by a specific injection system, preferably a periodic injection system, and the finely divided particles, such as droplets, formed in this way are conveyed by a carrier gas stream and introduced into the reaction chamber, where the deposition and growth of the carbon nanotubes or nitrogen-doped carbon nanotubes take place.