We reported an efficient method to prepare nylon-6 (PA6) graphene (NG) composites by in situ polymerization of caprolactam in the presence of graphene oxide (GO). During the polycondensation, GO was thermally reduced to graphene simultaneously. By adjusting the feed ratio of caprolactam to GO, various composites with 0.01-10 wt % content of graphene were obtained. The highly grafting nylon-6 arms on graphene sheets was confirmed by XPS, FTIR, TGA and AFM measurements, showing the grafting content up to 78 wt % and homogeneous 2D brush-like morphology from A FM observations. The efficient polymer-chain grafting makes the grapheme homogeneously dispersed in PA6 matrix and depresses the crystallization of PA6 chains. Furthermore, we prepared NG fibers by melt spinning process, and found that the tensile strength increased by 2.1 folds and Young's modulus increased by 2.4 folds with the graphene loading of 0.1 wt % only, revealing an excellent reinforcement to composites by graphene. The in situ condensation polymerization approach paves the way to prepare graphene-based nanocomposites of condensation polymers with high performances and novel functionalities.