Well-defined nanostructural control from biological motifs is gaining attention among materials scientists. We are reporting that the β-sheet structure of L-polyalanine plays a critical role in developing a fibrous nanostructure as well as the sol-to-gel transition of amphiphilic poly(ethylene glycol)-L/or DL-polyalanine diblock copolymers. L-isomers underwent transitions from random coils to β-sheets, and to nanofibers as the polymer concentration increased, whereas the DL-isomer remained as a random coil structure without developing any specific nanostructure. At high polymer concentrations, the aqueous polymer solutions underwent a sol-to-gel transition as the temperature increased, a so called reverse thermal gelation. The L-isomer with a preassembled β-sheet secondary structure facilitates the sol-to-gel transition rather than the DL-isomer with a random coil structure. Thus, only the L-isomer showed a sol-to-gel transition in the physiologically important range of 20–40 °C. This report provides fundamental information on the relationship between hierarchical structures of polypeptides and the thermosensitive sol-gel transition of the polypeptide aqueous solution.