Polyamide 6 (PA6) and stabilized PA6 with two different thermal stabilizers, (ie, metal salts CuI/KI and hindered amine KYN818 [1,3-benzendicarboxamide,N,N'-bis(2,2,6,6-tetramethyl-4-piperidinyl)]) were separately prepared by melt blending and then aged for different time. The effects of aging temperature and aging time on crystallization behaviors of PA6 and stabilized PA6 were systematically investigated by differential scanning calorimetry and X-ray diffraction. The variations of melting temperature, crystallinity and 2 theta values suggest that aging under high temperature will accelerate the formation of crystals initially and then the degradation of molecular chains. The yellowness index and maximum decomposition temperature of stabilized PA6 are higher than that of PA6, which is dominated by different aging mechanisms and properties of stabilizers. In addition, the viscosity of all systems is increased at the preliminary stage, which is caused by the crosslinking network and the post-polycondensation. The results of dynamic mechanical analysis showed that the glass transition temperature (T-g) decreases because the degradation of molecular chains during the aging process. Moreover, the carbonyl index for stabilized PA6 is lower than PA6, indicating the degradation is hindered by stabilizers. A comparison of the stabilization performance between CuI/KI and KYN818 was investigated to differentiate the long-term stabilizing efficiency.