A detailed analysis of the IR spectrum of melt-quenched PLLA-PEG-PLLA sample and its change with temperature is presented. It was found that, in the initial state of the melt-quenched PLLA-PEG-PLLA sample, the PLLA constituent does not exist as a normal amorphous or crystalline phase but as a kind of mesophase, which is confirmed by the WAXD data. The IR data suggest that both chain conformation and chain packing arc disordered to some extent in such a mesophase. Therefore, this mesophase is less thermally stable than the ordered a phase and even the disordered alpha phase (alpha') of PLLA. In the heating process, the mesophase of PLLA in the PLLA-PEG-PLLA copolymer starts the partial melt first upon heating in a stepwise way, and subsequently it is reorganized directly into the stable alpha phase but not through the alpha' phase of PLLA. These phase transition behaviors are examined and discussed at the molecular functional group level.