We transformed hydrophobic/hydrophobic styrene/butadiene/styrene tri-block copolymer (SBS) to hydrophobic / hydrophilic microphase-separated surfaces by grafting with hydrophilic poly(ethylene glycol) (PEG) on poly(butadiene) (PB) domain via hydrocarboxylation and hydrobromination and investigated the anti-thrombogenicity of these surfaces. In the case of SBS cast film from toluene solution, PEG was densely grafted because of the development of an unevenness on the order of several 10 nm on the surface, which had a huge surface area in comparison with poly(butadiene) rubber with its uniformly smooth surface. Grafted PEG (molecular weight ¼ 600) was found to clearly inhibit adhesion and activation of platelets and coagulation of the whole blood component, which is indicative of anti-thrombogenicity. These properties correspond to a surface coated by a copolymer of 2-methacryloyl-oxyethyl phosphorylcholine and n-butyl methacrylate, which is well known to be the best excellent anti-thrombogenic material in the world. Melt-molded SBS film, which also has an uneveness on the order of several 10 nm, showed similar excellent anti-thrombogenicity.