1. A method for making a thin film transistor, the method comprising of: (a) providing a plurality of carbon nanotubes and an insulating substrate; (b) flocculating the plurality of carbon nanotubes to acquire a carbon nanotube structure; (c) applying the carbon nanotube structure on the insulating substrate; (d) forming a source electrode, a drain electrode, and a gate electrode; and (e) covering the carbon nanotube structure with an insulating layer, wherein the source electrode and the drain electrode are connected to the carbon nanotube structure; and the gate electrode is electrically insulated from the carbon nanotube structure by the insulating layer, wherein step (a) further comprises following sub steps of: (a1) providing a substantially flat and smooth base; (a2) forming a catalyst layer on the substantially flat and smooth base; (a3) annealing the substantially flat and smooth base with the catalyst layer in air at a temperature in a range from about 700° C. to about 900° C. for about 30 to 90 minutes; (a4) heating the substantially flat and smooth base with the catalyst layer to a temperature in a range from about 400° C. to about 740° C. in a furnace with a protective gas therein; (a5) supplying a carbon source gas to the furnace for about 5 to about 30 minutes and growing the plurality of carbon nanotubes on the substantially flat and smooth base; and (a6) separating the plurality of carbon nanotubes from the substantially flat and smooth base.