An extruded composite structural artificial lumber product is manufactured from wood fiber and polyethylene, including recycled polyethylene, by reducing the wood fiber and polyethylene each to a finely divided particles, mechanically mixing the wood fiber and polyethylene particles together with a measured amount of a powdered endothermic foaming agent, mechanically mixing the mixture, without pre-pelletization, in a twin-screw extruder where it is compressed and heated until it becomes plastic and homogenous, and then extruding the plastic mixture, using vacuum to remove vaporized moisture and other excess volatiles, through a molding die which forms structural profile of the desired product. The endothermic foaming agent ingredient causes greater expansion in the center of the extruded profile, thereby compressing a greater proportion of plastic material against the sides of the extrusion die, increasing the density and strength on the outside of the extrusion, and reducing the density (with no significant loss of overall strength) on the inside. The resulting extruded artificial lumber product has a specific gravity of about 1.0, plus or minus 20%, with no significant variation in dimensions after cooling.