Polyamide 6 (PA6) fibers were coated with polycarbonate-based thermoplastic polyurethanes (CPUs) by dip-coating method and cured at 100 degrees C for 24 h. There were abrupt increases in tensile strength and elongation at break of the CPU-coated PA6 fibers. This was due to the increase of crystallinity from the structural changes inside the fiber induced with the CPU coating. An interfacial interaction region of 10 mu m was formed at the interface of each PA6 fiber and the CPU-thin film coating. Interface stiffening occurred when the PA6 fiber was under mechanical stress and the CPU coating delayed the formation of microcracks on the same under loaded stress. All these factors resulted in the increase of strength and elongation of CPU-coated PA6 fibers. CPUs showed good adhesion to PA6 fibers. Blending has been the key to the performance enhancement of PA6 fibers, but the present method of thin film CPU coating improved their performance without changing the inherent morphological properties. POLYM. ENG. SCI., 49:1970-1978, 2009. (C) 2009 Society of Plastics Engineers