Poly(ethylene glycol)-conjugated polypeptides have been drawing attention as a biomaterial as well as a pharmaceutical agent. In this paper, we synthesized a series of poly(ethylene glycol)-poly(l-alanine) block copolymers (PEG-l-PA) and investigated the block length effect on (1) the secondary structure of the PA, (2) the nanostructure of the self-assembled amphiphilic PEG-l-PA, and (3) the thermosensitivity of the PEG-l-PA aqueous solution. First, the molecular weight of the l-PA was fixed at 700-760 Daltons and that of the conjugated PEG varied over 1,000, 2,000, and 5,000 Daltons. l-PA with an antiparallel β-sheet structure in water transformed into an α-helical structure, and the self-assembled nanostructure of PEG-l-PA changed from a fibrous structure to a spherical micellar structure as the molecular weight of conjugated PEG increased. Then, when the molecular weight changed from 700 to 1,500 Daltons at a fixed molecular weight of PEG at 2,000, similar transitions involving antiparallel β-sheets changing to α-helices, and fibers to spherical micelles were observed. The polymer aqueous solution underwent a sol-to-gel transition as the temperature increased in a high polymer concentration range of 3-14 wt%. Interestingly, the transition temperature did not follow the simple rule that a more hydrophobic polymer has a lower transition temperature. This paper suggests that the control of PEG molecular weight in PEG-conjugated polypeptide biomaterials is important in that it affects the secondary structure of the polypeptide, the nanoassembled morphology, and the thermosensitivity of the polymer.