Nylon 6/carboxylic acid-functionalized silica nanoparticles (SiO2-COOH) nanocomposites were prepared by in situ polymerization of caprolactam in the presence of SiO2-COOH. The aim of this work was to study the effect of carboxylic silica on the properties of the nylon 6 through the interfacial interactions between the SiO2-COOH nanoparticles and the nylon 6 matrix. For comparison, pure nylon 6, nylon 6/SiO2 (unmodified) and nylon 6/amino-functionalized SiO2 (SiO2-NH2) were also prepared via the same method. Fourier transform infrared spectrometer (FTIR) spectroscopy was used to evaluate the structure of SiO2-COOH and nylon 6/SiO2-COOH. The results from thermal gravimetric analysis (TGA) indicated that decomposition temperatures of nylon 6/SiO2-COOH nanocomposites at the 5 wt % of the total weight loss were higher than the pure nylon 6. Differential scanning calorimeter (DSC) studies showed that the melting point (T-m) and degree of crystallinity (X-c) of nylon 6/SiO2-COOH were lower than the pure nylon 6. Mechanical properties results of the nanocomposites showed that nylon 6 with incorporation of SiO2-COOH had better mechanical properties than that of pure nylon 6, nylon 6/SiO2, and nylon 6/SiO2-NH2. The morphology of SiO2, SiO2-NH2, and SiO2-COOH nanoparticles in nylon 6 matrix was observed using SEM measurements. The results revealed that the dispersion of SiO2-COOH nanoparticles in nylon 6 matrix was better than SiO2 and SiO2-NH2 nanoparticles. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 1316-1324, 2011