AbstractNatural fiber–biopolymer composites have been prepared using flax and poly(3‐hydroxylbutyrate) bipolyesters (PHB). The biopolyesters consist of the homopolyester PHB and its copolymers with 5 and 12% 3‐hydroxyvalerate (PHV). These biopolymer–natural fiber composites provide structures totally composed of biodegradable and renewable resources. The adhesion between the fibers and the polyesters was better than for analogous polypropylene composites. Wetting of the fibers by the polyesters was observed using scanning electron microscopy. The composites were limited by the properties of the polyesters. PHB is a brittle polymer though flexibility is improved in its copolymers with PHV, but at the expense of crystallization rate. Nucleation was increased by the fibers and silane coupling agent used as adhesion promoter. The melting temperature was influenced by the promoted adhesion and copolymerization. The bending modulus was increased in the composites and dynamic mechanical analysis provided storage modulus of as much as 4 GPa at 25°C with a smaller component as the loss modulus. The maximum in the loss modulus curve was taken as the glass transition temperature, and this increased in the composites. The influence of silane coupling agent was found beneficial for the material properties of the biopolyester–flax composites. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 2114–2121, 2004