Thermoplastic polyurethane elastomers (TPUs) from diol-terminated poly(tetramethylene glycol) (PTMG), 1,4-butanediol (BD) and 4,4’-diphenylmethane diisocyanate (MDI) were modified by copolymerizing with diamine-terminated poly(gamma-methyl-L-glutamate) oligomer (gamma-PMG). In TPUs modified by the gamma-PMG segment, the hard segment showed a decreased crystallinity and a melting point depression, as some of the hard segments were replaced by the gamma-PMG segment. This behaviour is probably due to the strong interaction and phase mixing between gamma-PMG and hard segments. The melting behaviour of the PTMG segment suggested a weaker interaction between gamma-PMG and PTMG segments compared with that between hard segments and PTMG segments. When a small amount of hard segments was replaced by gamma-PMG segments, the modulus increased and the elongation at break decreased.