Poly(L-lactic acid) (PLLA) pellets were degraded in a batch reactor under hydrothermal conditions of aqueous sodium hydroxide Solution, from 343 to 453 K, compared to water in the absence of NaOH. Over ail entire range of conversion, Components solubilized from PLLA mainly consisted Or L-lactic acid (LLA) in the presence of NaOH. For instance, PLLA was nearly completely converted into LLA at 433 K for 60 min or 453 K for 20 min with 0.6 M aqueous NaOH solution, where D-lactic acid was not observed. The degradation reaction with/without NaOH proceeded in the induction stage followed by the major degradation stage, as has been seen for polyesters reported in our previous Studies. In the major degradation stage the overall reaction rate for PLLA was represented by 2/3-order reaction kinetics with respect to the amount of unreacted polymer, suggesting that the reaction occurred on the polymer Surface. The NaOH concentrations above 0.6 M at 343 K hardly affected the reaction rates, but did affect the induction time remarkably. It can be considered that the degradation reaction is controlled by not chemical reaction but dissolution of products on the pellet surface.