Abstract: Multi-walled carbon nanotubes(MWCNTs) were functionalized with 4-substituted benzoic acid via direct Friedel-Crafts acylation in a mild reaction medium of polyphosphoric acid(PPA)/pentoxide(P₂O₅). Functionalized multi-walled carbon nanotubes (F-MWCNTs)/polyamide 6(PA6) composite was prepared via in situ polymerization and the dried powders were melt-spun into fibers. The fibers were characterized using TEM, TG, DSC, SEM. MWCNTs inherently have functional groups on their surfaces as a result of the reaction. F-MWCNTs are homogeneously dispersed in PA6 matrix without any aggregation. The incorporation of F-MWCNTs into PA6 has no obvious effect on the melting point and the crystallinity of PA6, except increasing the crystallization temperature. With the content of F-MWCNTs increasing, thermal stability of the composite fibers is improved significantly. The tensile strength and Young’s modulus of the fibers are significantly improved with increasing the content of F-MWCNTs. The tensile strength and Young’s modulus of the fibers reach a maximum when the mass fraction of F-MWCNTs is 0.5%, which are improved by 45% and 208% respectively than PA6 fibers.