A thermionic emission device includes an insulating substrate, a patterned carbon nanotube film structure, a positive electrode and a negative electrode. The insulating substrate includes a surface. The surface includes an edge. The patterned carbon nanotube film structure is partially arranged on the surface of the insulating substrate. The patterned carbon nanotube film structure includes two strip-shaped arms joined at one end to form a tip portion protruded from the edge of the surface of the insulating substrate and suspended. The patterned carbon nanotube film structure includes a number of carbon nanotubes parallel to the surface of the insulating substrate. The patterned carbon nanotube film structure is connected between the positive electrode and the negative electrode in series.