Woven flax reinforced and woven glass roving reinforced unsaturated polyester composites were fabricated at various fibre volume fractions. At equivalent fibre volume fractions, the warp tensile and flexural properties, as well as the Charpy impact strengths of both types of reinforced composites were compared. It was found that warp flexural and tensile strengths of woven flax reinforced composites were lower than the strengths exhibited by the woven glass composites. The specific stiffness of woven flax composites were also observed to be of a lower value than the glass reinforced composites specific moduli. The Charpy impact strengths exhibited by the glass composites were also a higher value. The Tex (size) of the weft yarns within woven flax fabric were investigated to establish how they affect the warp and weft mechanical properties (flexural, tens ile and Charpy impact strength) of epoxy composites when the flax fabric was used as reinforcement in two simple geometries. It was found that the warp and weft mechanical properties of woven flax fabric reinforced epoxy composites were influenced by the Tex of the weft yarn reinforcement. The stacking sequence used to arrange the woven flax reinforcement also influenced the mechanica l properties of the composites. In addition, epoxy composites were fabricated containing I 2 different types of woven flax fabric. An investigation was performed to establish the influence the weave type architecture of woven flax reinforcement has upon the flexural properties and Charpy impact strength of epoxy composites. The warp and weft flexural properties and Charpy impact strengths were observed to be significantly different in composites reinforced with plies of woven flax fabric that consisted of highly crimped warp yarns. It was also observed that the weave type of woven flax fabric reinforcement used in composites did affect their warp flexural properties, this was dependent upon the frequency of warp yarn crimping. A study was undertaken to gain an understanding of the micromechanical processes that cause non-linear behaviour in flax fibre reinforced unsaturated polyester composites. Unidirectional composite bars reinforced with high quality flax sliver were fabricated at various fibre volume fractions. Tensile, flexural and Charpy impact tests were performed. Composite properties exhibited a linear rule of mixtures relationship with fibre content up to a fibre volume fraction of 60%. The response to straining of flax reinforced unidirectional composites bars under tensile loading conditions were recorded and analysed. Though exploring the deformation behaviour of flax composites via loading and unloading behaviour and acoustic emissions analysis, it was found that they undergo yielding at low values of stress and strain and that the reinforcing fibres are at the route of this behaviour. The existence of micro-compressive defects along the lengths of flax fibres and the effect the defects have upon stress-transfer are the likely cause of the observed deformation behaviour.