The influence of duration of mechanical activation of amorphous carbon produced by plant material pyrolysis on the morphology of moldable multilayer carbon nanotubes is studied. The maximum nanotube yield is observed when amorphous carbon is mechanically activated for 36 hours. The nanotube yield depends a great deal on the type of plant materials submitted to pyrolysis. It is demonstrated that prolonged mechanical activation of carbon composite in a vario-planetary mill leads to formation of aggregates consisting of multilayer nanotubes and amorphous carbon and to loss of thermal stability of nanotubes. This fact is responsible for the decrease in carbon nanotube content in products of vacuum annealing carried out for nanotube purification.