An apparatus for manufacturing carbon nanotubes includes: a reaction chamber having an inlet at a bottom and an opposite outlet at a top thereof, and a substrate region configured for accommodating a substrate for growing carbon nanotubes thereon; an electric field generating device configured for generating an electric field around the substrate region, the electric field being substantially perpendicular to the substrate; and a magnetic field generating device configured for generating a magnetic field around the substrate region, the magnetic field being substantially perpendicular to the substrate.