The long-term stressthermooxidative aging behavior of polyamide 6 (PA6) was studied in terms of the creep behavior, mechanical properties, chemical structure, crystallization, and orientation behavior. During aging, a thermooxidation reaction occurred, which included molecular chain degradation and crosslinking, in PA6. Meanwhile, when the samples were subjected to stress, crystallization, orientation, and chain scission were induced. In the initial stages of aging, the stress-induced crystallization and orientation dominated; this resulted in an increase in the creep deformation, mechanical strength, crystallinity, and orientation factor. Molecular degradation and chain scission dominated in the subsequent aging process and resulted in a decrease of the mechanical strength, reduced viscosity, crystallinity, and orientation factor and an increase in the formation of oxide and peroxide products. The stress may have promoted the chain scission of PA6 during thermal aging and resulted in a decrease in the reduced viscosity and an increase in the carboxylic acid concentration. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013