Abstract: The present invention relates to a method for manufacturing carbon nanotubes comprising: a preparatory step of a supported catalyst; a temperature-raising step of inserting the supported catalyst into a reactor, injecting hydrocarbon gas and hydrogen gas at the same time, and raising the temperature of the reactor to between 900 to 1000° C. to synthesize carbon nanotubes; and a temperature-lowering step of lowering the temperature of the reactor to between a room temperature to 200° C., injecting only hydrogen gas, and synthesizing carbon nanotubes. The carbon nanotubes manufactured by the above method have high purity, and excellent selectivity for double wall carbon nanotubes can be achieved.