Amphiphilic block polypeptides of [GRAPHIC] poly(sarcosine)-b-(L-Val-Aib)(6) and poly(sarcosine)-b-(L-Leu-Aib)(6) and their steteoisomers 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 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 coinbinations,of the Leu- and Val-based amphiphilic polypeptides. The membrane elasticity is found to be adjustable by a suitable combination of helical brocks in terms of stereocomplex formation and the interdigitation of side chains among helices in the molecular assemblies.