A method of fabricating a composite field emission source is provided. A first stage of film-forming process is performed by using RF magnetron sputtering, so as to form a nano structure film on a substrate, in which the nano structure film is a petal-like structure composed of a plurality of nano graphite walls. Afterward, a second stage of film-forming process is performed for increasing carbon accumulation amount on the nano structure film. Therefore, the composite field emission source with high strength and nano coral-like structures can be obtained, whereby improving the effect and life of electric field emission.