A method for making a thin film transistor, the method comprising the steps of: providing an insulating substrate; forming a carbon nanotube layer on the insulating substrate, the carbon nanotube layer includes a plurality of carbon nanotubes; applying a source electrode and a drain electrode spaced from each other and electrically connected to two opposite ends of at least one of carbon nanotubes; covering the carbon nanotube layer with an insulating layer; and placing a gate electrode on the insulating layer, the gate electrode being opposite to and electrically insulated from the carbon nanotube layer by the insulating layer.