Graphene fibers made from a graphene film formed into an elongated fiber-like shape and composite materials made from the graphene fibers. The elongated fiber-like shape may be the graphene film in a rolled spiral orientation or the graphene film in a twisted formation. The graphene film has benzoxazine groups formed on at least an outer surface of the graphene film and epoxide groups formed on at least one edge of the graphene film. Methods of increasing strength of a composite material include combining a resin matrix with a plurality of the graphene fibers to form a prepreg material and curing the prepreg material to form the composite material.