Carbon nanofibers (CNFs) were coated with an ultrathin layer of poly(acrylic acid) (PAA) via plasma polymerization. The effect of the plasma reactor parameters on the extent of the CNF modification was studied. SEM micrographs showed that surface roughness increased with the plasma treatment. The thickness of the ultrathin PAA layer deposited on the CNF was determined by STEM to be ca. 8 nm. Untreated and treated CNF were melt-mixed with polyamide 6 (PA6) in a Brabender mixing chamber to obtain PA6/CNF nanocomposites. The effect of the plasma treatment on the dispersion and compatibility was examined and found to improve markedly. Fractured tensile specimens showed that the CNF seemed to be completely embedded in the polymer matrix, indicating high compatibility between the PA6 and the PAA-coated CNF. Tensile stress and tensile modulus of PA6 nanocomposites with treated CNF were found to increase by 30 and 48%, respectively, when compared with those with untreated CNF. However, the increase in tensile stress and modulus with respect to pure PA6 was 52 and 88%, respectively. Finally, XRD showed that the CNF induce the formation of the alpha (alpha)-crystalline phase in PA6. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 3510-3518, 2009