To improve the effective thermal conductivity of hexagonal boron nitride (h-BN) in polymer matrix, surface modification upon it could be a usual and efficient method. Boron nitride nanosheets (BNNSs) were abundantly decorated by amino groups via a two-step reaction. The prehydroxylation of BNNSs in a concentrated base environment could increase the active reaction sites on the surface of BNNSs, so that aniline carbocations produced by the hydrolysis of the diazonium salt were utilized to achieve a high grafting content of aniline groups onto the surface of modified BNNSs (A-BNNSs). During melt blending, chemical reactions between A-BNNSs and the carboxyl groups of PA6 chains benefited the interfacial force between A-BNNSs and PA6 matrix; thus, the distribution of A-BNNSs in PA6 matrix was enhanced. Consequently, the prepared PA6/A-BNNSs nanocomposites had improved thermal conductivity, better thermal stability, and enhanced mechanical properties compared with the PA6/BNNSs nanocomposites at the same filler content. In particular, as for PA6/A-BNNSs nanocomposites with 20 wt % filler loading, its thermal conductivity was 2.886 W.m(-1).K-1, nearly 10 times that of pure PA6, and its initial decomposition temperature was nearly 100 degrees C higher than that of pure PA6, which would be a popular candidate for the thermal conductivity field.