The thermosensitivity of biodegradable and non-toxic amphiphilic polymer derived from a naturally occurring polypeptide and a derivative of amino acid was first reported. The amphiphilic polymer consisted of poly(γ-glutamic acid) (γ-PGA) as a hydrophilic backbone, and L-phenylalanine ethyl ester (L-PAE) as a hydrophobic branch. Poly(γ-glutamic acid)-graft-L-phenylalanine (γ-PGA-graft-L-PAE) with grafting degrees of 7–49% were prepared by varying the content of a water-soluble carbodiimide (WSC). γ-PGA-graft-L-PAE with a grafting degree of 49% exhibited thermoresponsive phase transition behavior in an aqueous solution at around 80°C. The copolymers with grafting degrees in the range of 30–49% showed thermoresponsive properties in NaCl solution. A clouding temperature (Tcloud) could be adjusted by changing the polymer concentration and/or NaCl concentration. The thermoresponsive behavior was reversible. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012