Amphiphilic block polypeptides of poly(sarcosine)-b-(l-Val-Aib)6 and poly(sarcosine)-b-(l-Leu-Aib)6 and their stereoisomers were self-assembled in water. Three kinds of binary systems of poly(sarcosine)-b-(l-Leu-Aib)6 with poly(sarcosine)-b-poly(d-Leu-Aib)6, poly(sarcosine)-b-poly(l-Val-Aib)6, or poly(sarcosine)-b-(d-Val-Aib)6 generated vesicles of ca. 200 nm diameter. The viscoelasticity of the vesicle membranes was evaluated by the nanoindentation method using AFM in water. The elasticity of the poly(sarcosine)-b-(l-Leu-Aib)6/poly(sarcosine)-b-poly(d-Leu-Aib)6 vesicle was 11-fold higher than that of the egg yolk liposome but decreased in combinations of the Leu- and Val-based amphiphilic polypeptides. The membrane elasticity is found to be adjustable by a suitable combination of helical blocks in terms of stereocomplex formation and the interdigitation of side chains among helices in the molecular assemblies.