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The surface of alumina (Al2O3) particles was modified by a blocked isocyanate silane coupling agent, which was synthesized by using methylethyl ketoxime (MEKO) to block isocyanate-propyltriethoxy silane via nucleophilic reactions. For comparison, the surface of Al2O3 particles was also modified by propyltrimethoxy silane (KH-331). The thermally conductive and electrical insulating Al2O3/polyamide 6 composites were prepared by dispersing modified Al2O3 in polyamide 6 (PA6) using a twin-screw extruder. The X-ray photoelectron spectroscopy (XPS) results revealed that blocked isocyanate-propyltriethoxy silane and KH-331 were successfully attached to the surface of Al2O3 particles with chemical bonds. The blocked isocyanate silane modified Al2O3 were well dispersed in PA6 as revealed by scanning electron microscopy (SEM). When the Al2O3 content was 70 wt%, the tensile strength (85.2 MPa) and flexural strength (136.4 MPa) of blocked isocyanate silane modified Al2O3/PA6 composites were higher than those of KH-331 modified Al2O3/PA6 composites (52.5 MPa, 77.8 MPa), respectively. The thermal conductivity and impact strength of blocked isocyanate silane modified Al2O3/PA6 composites were also higher than that of KH-331 modified Al2O3/PA6 composites at the same content of Al2O3. The volume electrical resistivity of modified Al2O3/PA6 composites decreased with increasing Al2O3 content, but they still were good electrical insulating materials.