Silk fibroin was dissolved in calcium chloride/ethanol/water mixture(1/2/8 in mole ratio)at 7$0^{circ}C$ for 4hrs. The dissolved silk fibroin was regenerated by casting the dialyzed solution into the film. The films were treated with 50% aqueous methanol solution for 15 min. Then the films were graft-copolymerized with methylmethacrylate and styrene by methane plasma initiation and their antithrombogenicities were investigated by the methods of blood clotting and platelet adhesion tests. The results from the two "in vitro" tests were analyzed in connection with surface tension of the film. It was found that the antithrombogenicity of films was improved by the surface grafting and varid with the fraction of polar component of surface tension to the total surface tension of film.n of film.