A hybridization of structurally regulated biopolymers and conventional synthetic polymers offers promising opportunities to design novel polymeric nanomaterials. In this study, we newly prepared an amphiphilic triblock copolymer with beta-sheet forming peptide as a central block, polystyrene-block-tetra(leucine)-block-poly(ethylene glycol) (PS-L-4-PEG), by combining the solid phase peptide synthesis with the atom transfer radical polymerization (ATRP). On the basis of several morphological and structural analyses using atomic force microscopy, transmission electron microscopy, FTIR spectroscopy, and contact angle measurement, the PS-L-4-PEG was found to form PEG-shell spherical and/or elliptical vesicles with a diameter of 30-100 nm in aqueous medium. By contrast, the PS-L-4-PEG self-assembled into PS-shell spherical aggregates in toluene, which is good solvent for both PEG and PS. In both cases, the central peptide formed a beta-sheet network in the nanoassemblies. Furthermore, the PS-L-4-PEG was found to stabilize a water-in-oil emulsion remarkably in comparison with the PS-PEG diblock copolymer, demonstrating the potential of this peptide-polymer hybrid as a useful polymeric surfactant.