The aqueous solution of poly(L-Ala-co-L-Phe)-poly(propylene glycol)-poly(ethylene glycol)-poly(propylene glycol)-poly(L-Ala-co-L-Phe) block copolymers (PAF-PLX-PAF) in a concentration range of 6.0-10.0 wt % underwent sol-to-gel transition as the temperature increased from 10 to 50 degrees C. Circular dichroism spectra, hydrophobic dye solubilization, dynamic light scattering, and transmission electron microscopy image of the polymer suggest that the polymers form micelles in water, where the hydrophilic (PLX) blocks form a shell and the hydrophobic (PAF) blocks form a core of the micelle. Circular dichroism, FTIR, and (13)C NMR spectra suggest that sol-to-gel transition accompanies partial strengthening of the beta-sheet structure of PAF and a decrease in molecular motion of the PLX. The sol-to-gel transition temperature could be controlled by varying the molecular weight of PAF and PLX blocks, the ratio of Ala to Phe, and the corresponding secondary structure of the polypeptide.