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The present study on the use of flax stems as reinforcement for green composites has three major objectives: 1) to use a variety of flax normally cultivated for its seeds, not for its fibers, in a textile-related application; 2) to avoid the wastage and pollution resulting from the traditional burning of these stems; and 3) to incorporate them almost directly in composites. First seed-flax stems were cut the same length and washed with hot water.Their outer fibrous layer was sometimes peeled so that only the wooden inner part would remain.Two types of reinforcement were thus prepared: wooden parts only (WOOD) and whole stems (STEM).Reinforcement pieces were then aligned parallel to each other, maintained together by Kevlar threads, and hot pressed (sometimes in a frame) between polypropylene PP or low-density polyethylene LDPE films to make laminae and laminates.The procedure followed was progressively improved by trying different processing settings such as temperature, pressure, and press time, and by changing such parameters as the arrangement of stems, the number of polymer films, etc. Finally samples were cut in the composites and subjected to tensile and bending tests.The results obtained showed that STEM composites exhibit higher mechanical properties due to the presence of fibers in the outer layer of the reinforcement pieces.Tensile strength and modulus are respectively comprised between 20-35 MPa and 3.2-6.7 GPa.The final behavior depends much on the orientation of reinforcement pieces in the composites, the relative quantity of polymer and the processing conditions, such as temperature and the use of a frame.For LDPE laminates made with a frame, the bending strength (60-70 MPa) was much increased compare to pure LDPE (10-40 MPa) and three-layered PMDI wood particle boards reinforced withflax straw mats (25-40 Mpa).