Method of patterning thin film

A method of patterning a thin film includes steps as follows. The thin film is formed. The thin film includes a plurality of first molecules, and each of the first molecules has a conjugated structure. A mask is covered on the thin film. The mask includes at least one exposing area, and the exposing area is correspondent to an illuminated region of the thin film. A solvent annealing and illuminating step is conducted, wherein the thin film covered by the mask is illuminated with a light source under an atmosphere of a first solvent, and a wavelength range of the light source is correspondent to an energy enabling the first molecules to reach an excited state. Thus a thickness of the illuminated region of the thin film is increased or decreased so as to form a pattern on the thin film.