In order to improve the mechanical property of nylon 6 fiber, the aminated graphene oxide (A-GO) was modified with dodecylamine (DA). The A-GO/PA6 composite fibers were fabricated at various mass ratios by melt spinning. Through the characterization of A-GO and A-GO/PA6 composite fibers, the results indicate that the DA is successfully grafted onto GO and the layer spacing of A-GO is increased by 0.37 nm as compared with GO. A-GO has good interface interaction with the PA6 matrix, which can achieve more uniform dispersion. A-GO plays a role of heterogeneous nucleating agent in the crystallization of PA6 fibers, which promotes the crystallization of nylon 6 fibers and induces the crystal form transformation from α form to γ form. Comparing to pure PA6 fiber, the 0.5% thermal mass loss temperature of A-GO/PA6 composite fibers is enhanced by 6.1～8.9 ℃, manifested a well thermal stability. When the content of A-GO is as low as 0.1%, the tensile strength of the composite fiber is improved to 1.4 times of pure PA6 fiber.