Ductility collapse is a stubborn issue for developing high-strength aluminum (Al) matrix composites reinforced with graphene nanosheets (GNSs). In order to solve this problem, here we introduced large amount of in-situ alumina nanoparticles (∼3 vol%) into the GNSs/Al composite to boost its dimple fracture capability. Microstructural characterization showed that the introduced alumina nanoparticles were α-Al₂O₃ with a diameter of ∼30 nm, which were distributed uniformly on the Al matrix. Tensile tests revealed that the composite possesses a high tensile strength of 464 MPa and appreciable amount of ductility (8.9%), the combination of which was superior to those in literatures. Fractographic analysis suggested that the in-situ alumina nanoparticles acted as nucleation sites for dimples and enabled strain-delocalization, leading to enhanced plastic deformability. An analysis on strengthening mechanisms further confirmed the synergistic reinforcing effect of ex-situ GNSs and in-situ alumina nanoparticles. These findings may provide guidance for promoting the mechanical properties of GNSs/Al composites.