The effect of chain packing on tensile properties was studied employing amorphous poly(lactic acid) PLA. It was found that the samples cooled in the temperature range from 60 to 80 °C, that is, slightly higher than the glass transition temperature Tg, showed ductile behavior with a low brittle-ductile transition temperature. Furthermore, the samples obtained by prolonged cooling at 56 °C also showed ductile behavior, whereas a shorter cooling time at the same temperature provided a brittle product. Even for the samples quenched at 40 °C, they showed ductile behavior after the exposure to postprocessing annealing operation at 60 °C; that is, the strain at break is larger than 3. This is an anomalous phenomenon for a glassy polymer. The dynamic mechanical analysis and thermal characterization revealed that the ductile samples show slightly higher Tg than the brittle ones, presumably due to high packing density of polymer chains. Moreover, it was found from infrared spectroscopy that the ductile samples show strong absorbance at 1267 cm(-1), ascribed to high energy gauche-gauche gg conformers. Following the classic Robertson's descriptions of plastic flow, it is concluded that the increase in the gauche-gauche gg conformers, which shows the conformation change under a low stress level, reduces the critical onset stress for shear yielding. The results demonstrated that the mechanical toughness of PLA can be controlled by the cooling conditions during processing and the postprocessing annealing operation.