The purpose of this study was the clarification of mechanisms where the introduction of photo-dissociable protecting groups affects mechanical and hydrolysis properties of PLA, by combination of experiments and molecular simulation, aiming to obtain a guideline to design the optimum introduction condition. First, the mechanism of decreasing in elastic modulus of PLA by introduction of protecting groups was discussed using a molecular dynamics simulation. As a result, it was suggested that the deflection of helical molecular structure of PLA is one of the main reasons. Secondary, the change in introduction efficiency of protecting groups was experimentally evaluated using a FT-IR. Here, the temperature of introduction process and the amount of protecting groups was varied. In addition, its mechanism was discussed based on a molecular orbital calculation. As results, it was suggested that the introduction efficiency of protecting groups into carboxyl end groups of PLA is determined by the relation between the total energy of the reaction field and the activation energy between protecting groups and reaction sites, resulting in elucidation of existing range of the optimum introduction condition.