Thermal degradations of poly(lactic acid), (PLA) and its fiber were studied via direct pyrolysis mass spectrometry. As the amount of PLA pyrolyzed was increased the relative yields of protonated and cyclic oligomers were increased indicating that inter-molecular interactions were promoted. Hydrolysis reactions caused significant decrease in the relative yields of high mass products. Intermolecular trans-esterifications were more effective during the pyrolysis of PLA fiber. On the other hand, PLA fiber showed lower sensitivity to water vapor. This behavior was associated with the increase in intermolecular trans-esterification reactions.