Provided is a method of modifying carbon nanotubes, the method including: preparing a mixed solution in which a radical initiator and a carbon nanotube are dispersed; applying energy to the mixed solution to decompose the radical initiator into a radical; and reacting the decomposed radical with a surface of the carbon nanotube, wherein the radical which has reacted with the carbon nanotube is detached from the carbon nanotube after the reaction with the carbon nanotube. In the method of modifying carbon nanotube, a radical is reacted with a carbon nanotube and then separated from the carbon nanotube to thus modify the surface of the carbon nanotube without chemical bonding. Accordingly, the conductivity of the carbon nanotube can be increased.