We report a thermosensitive polypeptide consisting of hydrophilic poly(ethylene glycol)s and hydrophobic phenyl alanine ethyl esters. It is soluble in most solvents 0 C and undergoes a micelle (radius approximate to 10 nm) to nanoparticle (radius approximate to 100 nm) transition in water as the temperature increases. The transition temperature could be controlled to E between 30 and 40degreesC by varying the number of poly(ethylene glycol) grafts. This system might be promising for advanced drug delivery.