Abstract In this article, the effects of crystal structures on polyamide 6(PA6) and low melting point polyamide 6(LPA6) were studied. After heat treatment, the crystal structures of PA6 and LPA6 changed. With the increase of annealed temperature, the crystallinity of PA6 and LPA6 increased and a γ-α crystal transformation occurred, the tensile and flexural properties of the both polyamide 6 increased, especially for LPA6. The Vicat softening temperature of LPA6 increased significantly while the Vicat softening temperature of PA6 decreased slightly. Keywords: Crystal structuresCrystallinityLow melting point polyamide 6Performance ACKNOWLEDGMENTS The authors acknowledge the Contract grant sponsor: the National Natural Science Foundation of China, Contract grant number: 50863001.