Polystyrene‐poly(butadiene‐co‐hydroxylated butadiene)‐polystyrene triblock copolymer (SHBS) with hydrophobic‐hydrophilic microdomain structure has been prepared through the hydroxylation of polybutadiene (PBD) block of anionically polymerized SBS triblock copolymer. X‐ray photoelectron spectroscopic and contact angle measurements revealed that the environmentally induced surface reorganization took place after exposure of the film to water in the case of low degree of hydroxylation of PBD block. The interaction between plasma protein and the SHBS surface has been studied on the basis of transmission electron microscopic observations of the specimen after immersing it in human serum albumin (HSA) and human fibrinogen (HFN) solutions. The adsorbed HSA and HFN were labeled with colloidal gold and the modified PBD block was stained with osmium tetroxide. Thus, the domain recognition of plasma protein can be analyzed. The amount of plasma protein adsorbed per unit area on PS domain did not depend on the degree of hydroxylation of PBD block. However, the amount of plasma protein adsorbed on the hydroxylated PBD block decreased with an increase in degree of hydroxylation. These behaviors can be ascribed to the selective protein adsorption onto hydrophobic phase in order to minimize the interfacial‐free energy between polymer surface and plasma protein solution.