A method for making thin film transistor includes: providing a gate electrode and forming an insulating layer on the gate electrode; providing a carbon nanotube film comprising a plurality of metallic carbon nanotubes and a plurality of semiconducting carbon nanotubes; laying the carbon nanotube film on a surface of the insulating layer, and placing the carbon nanotube film under a scanning electron microscope to take photo of the carbon nanotube film to distinguish the plurality of metallic carbon nanotubes and the plurality of semiconducting carbon nanotubes; removing the metallic carbon nanotubes, and forming a source electrode and a drain electrode on a surface of the semiconducting layer.