To improve the blood compatibility of Bombyx mori silk fibroin (SF) film, the film was modified by SO2 gas plasma treatment, or by a two-step process including NH3 gas plasma treatment and reaction with 1, 3-propane sultone. XPS and ATR-FTIR were used to analyze the surface chemical elements. In vitro antithrombogenicity was determined by the method of the activated partial thromboplastin time (APTT), prothrombin time (PT) and thrombin time (TT) tests. Percents of sulfur element on the surfaces of both modified SF films were 4.03% and 3.30%, respectively, while that of the control film was only 0.32%. Moreover, the antithrombogencity of treated films was increased remarkably due to surface sulfonation. The results implied a potential use of sulfonated SF for blood-contacting biomaterials.