A detailed analysis of the IR spectral changes of melt-quenched poly(l-lactic acid) (PLLA) sample is presented. It is found that the glass transition, cold crystallization, disorder-to-order and melting behavior of melt-quenched PLLA can be clearly revealed by choosing suitable characteristic bands and combing the 1st derivative manipulation. Of particular note, it is found that, there are chain segment relaxation for both the irregular conformation and the regular helical conformation in the process of glass transition. During the disorder-to-order phase transition, the content of 10/3 helix as indicated by the helical band at
921 cm−1 increases slightly, which suggests that the structural rearrangement and recrystallization may take place at the same time. These results provide more information to understand the glass transition and disorder-to-order phase transition behavior of PLLA at molecular level.