Poly (D, L-lactide) (PDLLA) degraded at processing temperature under air and nitrogen. A random chain scission model was established and used to determine the activation energy Ea, and FT-IR, 1H and 13C NMR were used to elucidate the degradation behavior under different atmospheres. Results showed that there were two to three stages. The 1st stage was dominated by the oligomers containing carboxylic acid groups and hydroxyl groups, during which oxygen and nitrogen had little effect on the degradation, thus they share similar Ea. When the oligomers were consumed over or evaporated, the 2nd stage began, and oxygen had a promoting effect on the thermo-oxidation process, resulting in the great decrease in Ea. The third stage of PDLLA was observed when it degraded under nitrogen over 200