A method for producing composite materials, such as thermoplastic resins with mineral and/or vegetable fillers, consisting in feeding a mineral and/or vegetable filler, preheating the filler, feeding a melted thermoplastic resin onto the filler, introducing the mixture of the filler and the thermoplastic resin in an extruder, subjecting the mixture to high compression, producing a high partial vacuum, and compressing the mixture in an extrusion head, out of which the material to be subjected to subsequent treatments flows.