In this work. a starch-based biopolymer was reinforced with flax strands. Composite materials at different fiber content were obtained by means of high-shearing mixing followed by injection molding processing. The evaluation of the mechanical properties gave a significant increase of the strength and stiffness of composites with the percentage of flax strands. The enhancement was analyzed in terms of compatibility and extension of the adhesion at fiber-matrix interface. The intrinsic mechanical properties of the reinforcement were measured and the behavior of the obtained composites was studied in terms of the modified rule of mixtures and the modified Halpin-Tsai equations. (C) 2009 Elsevier Ltd. All rights reserved.