This article focuses on an improved method, i.e., improved in situ polymerization of epsilon-caprolactam in the presence of melamine derivatives to prepare flame-retardant melamine cyanurate/polyamide 6 (MCA/PA6) nanocomposites. The chemical structures of these synthetic flame retardant composites are characterized by Fourier-transform infrared spectroscopy and X-ray diffraction. Morphologies, mechanical properties, and thermal properties also are investigated by the use of transmission electron microscopy, mechanical testing apparatus, differential scanning calorimetry, and thermogravimetric analysis, respectively. Through transmission electron microscopy photographs, it can be found that the in situ-formed MCA nanoparticles with diametric size of less than 50 mu are nanoscaled, highly uniformly dispersed in the PA6 matrix. These nanocomposites, which have good mechanical properties, can reach UL-94 V-0 rating at 1.6-mm thickness even at a relatively low MCA loading level. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 2109-2116, 2009