A method for manufacturing carbon nanotubes with a desired length includes the steps of: providing an array of carbon nanotubes; placing a mask having at least an opening defined therein on the array of carbon nanotubes, with at least one portion of the array of carbon nanotubes being at least partially exposed through a corresponding opening of the mask; forming a protective film on at least one exposed portion of the array of carbon nanotubes; removing the mask from the array of the carbon nanotubes, with the carbon nanotubes being compartmentalized into at least a first portion covered by the protective film and at least one uncovered second portion; breaking/separating the first portion from the second portion of the array of the carbon nanotubes using a chemical method, thereby obtaining at least a carbon nanotube segment with a protective film covered thereon; and removing the protective film from the carbon nanotube segment.