The surface of poly[N,N’-(oxydiphenylene)pyromellitimide] film, Kapton, was modified by plasma graft polymerization of vinylimidazole to improve the adhesion with copper metal, and the surface of the modified Kapton film was investigated by means of XPS and atomic force microscopy. The plasma graft polymerization deposited the heterogeneous layer of poly(vinylimidazole) in the form of an island-sea type structure (about 200 nm height) at the surface of the Kapton film. The graft polymer layer interacted with copper metal to form a charge-transfer complex and contributed to improving the adhesion with copper metal. The peel strength of the adhesive joint between the Kapton film and copper metal was increased 2.9 times by the graft polymerization. The failure of the graft-polymerized Kapton film/copper metal adhesive joint did not occur at the interface between the graft polymer and metal, but mainly at the inner layer of the Kapton film.