This invention relates to a method and apparatus for controlling the length of a carbon nanotube, in cooperation with a substrate having at least one reference level on a surface of the substrate on which at least one carbon nanotube is formed, comprising at least one positioning platform for mounting and calibrating the substrate; a discharging electrode mounted on one side of the positioning platform to cut the carbon nanotube wherein the position of the discharging electrode can be calibrated with the positioning platform; a piezoelectric actuator for calibrating the position of the discharging electrode or the height of the discharging electrode relative to the substrate reference level; a position sensor for detection of the height of the substrate; and a voltage pulse supplying means for applying a voltage pulse to the discharging electrode to cut the carbon nanotube.