Flax fibres based composites are increasingly used as alternatives to glass fibres based ones mainly because of their low density, good mechanical properties and positive life cycle assessment. In contrast to glass fibres, flax fibres present a complex hierarchical and layered structure made of several biopolymers. Moreover, the production of flax fabrics from bast fibres to textile and composite applications requires several transformation steps involving mechanical processes and chemical treatments that have a significant influence on the biochemical composition, the structure and the properties of flax fibres, and hence strongly influence the microstructural and mechanical properties of biocomposites. As well, the processing techniques used for manufacturing flax fibres based composites influence greatly these properties. In a first part, this study compares the microstructure of thermoset polyester/flax fabrics composites processed through vacuum infusion and thermocompression. Results show that vacuum infusion produced lower fibre content in composites with lower matrix porosity and yarns packing. Mechanical properties are therefore lower but the fibers are less degraded because of the low pressure setting. The second part investigates the effect of different chemical extraction treatments on the flax fabrics properties (biochemical composition, fabrics dimension and density) and on the microstructure (reinforcing rate, porosity, bundles dispersion, crosslinking rate) of thermocompressed epoxy/flax fabrics composites. A gradient of solvation capacity of flax cell wall components was obtained with selected chemical extraction treatments. The analysis of flax yarns dispersion within composites showed that the chemical extraction treatments induced a pronounced individualization of the fibre bundles in elementary fibres and favored the diffusion of epoxy resin within the yarns during processing resulting in variations in fibre content and mechanical properties. Keywords: thermoset biocomposite, flax fabrics, vacuum infusion, thermocompression, chemical extraction treatment, fibre dispersion.