Abstract: A method of making a plastic reinforced by natural fibers which comprises natural fibers and a mixture prepared by combining per 100 parts by weight of an unsaturated polyester with 30-300 parts by weight of an organic foaming agent, 0.5-3.0 parts by weight of an organic peroxidase, and a predetermined quantity of a wood powder filling agent. The method includes the steps of (a) preparing a filling agent mixture by adding a predetermined quantity of a filling agent to a foaming agent; (b) preparing an unsaturated polyester mixture by adding the filling agent mixture of step (a) and a catalyst to the unsaturated polyester; and (c) impregnating the natural fibers in the unsaturated polyester mixture of step (b) before the natural fibers are arranged in a mold cavity, which is then heated under a pressure to bring about the production of the plastic reinforced by natural fibers.