Several types of poly(Gly-co-Pro) and poly(Ala-co-Pro) were synthesized. Their molecular conformations and thermosensitive properties were investigated in an attempt to find new thermosensitive materials. These polypeptides were assumed the polyproline-II structure in the temperature range of 20 to 80 degrees C. They also exhibited cloud points in light transmittance, indicating the phase transition. The transition temperatures increased with decrease in hydrophobicity of the polypeptides, that is, in the order poly(proline), poly(Ala-co-Pro) and poly(Gly-co-Pro).