New wood-synthetic resin composite products and methods of forming these products are provided. The products comprise a cellulosic or fibrous material and polypropylene (preferably reactor flake polypropylene). The products are formed by passing ingredients including the fibrous material and polypropylene through an extruder (preferably a twin screw extruder) at temperatures of from about 150-260° C. The extruded composite products have physical properties very similar to those of natural wood, including high tensile strengths, high compressive strengths, and high densities.