In order to make high performance wood-plastic composites (WPCs) from wood-fiber and mixed plastic wastes, virgin resins were compounded to simulate mixed plastic wastes, which included polypropylene, polyethylene and/or polystyrene, then grafted with maleic anhydride (MAH) by reactive extrusion. Highly filled WPCs were prepared by extruding. Mechanical testing results showed that the mechanical properties of the composites based on grafted virgin and waste plastics both significantly enhanced. The compatibility between the different plastics in the blend system and the interfacial adhesion between wood fibers and the blends were both improved with the modification of the blends, as evidenced by SEM. For the composites based on MAH grafted plastics, the water absorption and thickness swell decreased, which is true for the composite made from both virgin and recycled plastics. This blending-grafting modification method can be considered as a feasible approach to use mixed plastic wastes in the manufacture of high performance WPCs.