We report a chemical processing technology that allows the continuous spinning of single-walled carbon nanotubes (SWNTs)-nylon 6 (PA6) fibers by the in-situ polymerization of caprolactam in the presence of SWNTs, which simultaneously optimizes the morphology of the composite. We show that caprolactam is an excellent solvent for carboxylic-acid-functionalized SWNTs (SWNT-COOH) and that this allows the efficient dispersal of the SWNTs and subsequent grafting of PA6 chains to the SWNTs through condensation reactions between the carboxylic-acid group on SWNT-COOH and the terminal amine group of PA6. The existence of a graft copolymer between the PA6 chains and the SWNTs is demonstrated by IR, TGA, and AFM studies, and we show that the solubility of the polymerized material in formic acid is controlled by the degree of graft copolymerization. The amount of grafted PA6 chains that are attached to the SWNTs can be adjusted by controlling the concentration of the initiator (6-aminocaproic acid). The process leads to a uniform dispersion of the SWNTs, and the presence of the graft copolymer increases the polymer/SWNT compatibility while strengthening the interfacial interaction between the nanotube and matrix. The Young's modulus, tensile strength, and thermal stability of the SWNT-reinforced composite fibers produced by this process are significantly improved.