A theoretical model is suggested which describes a mechanism of the fracture toughness in a bimodal nanotwinned metal-graphene composite consisting of large grains with nanotwinned structure embedded into ultrafine-grained/nanocrystalline metal-matrix reinforced by graphene inclusions. In the framework of the model, the migration of nanotwin boundaries in the large grains releases in part local stresses near crack tips and provides the enhancement of the plastic deformation of the bimodal nanotwinned metal-graphene composites. At the same time, the presence of the graphene inclusions induces the crack bridging effect which also increases the fracture toughness of the metal-graphene composites. In exemplary case of aluminum-graphene composite, it was shown that the formation of the bimodal nanotwinned structure in ultrafine-grained/nanocrystalline matrix and account for the crack bridging by the graphene inclusions leads to a significant increase in the fracture toughness of the bimodal nanotwinned metal-graphene composites