In this article, three bio-composites, i.e. flax, linen and bamboo fabric reinforced epoxy resin, were manufactured using a vacuum bagging technique. The influence of alkali treatment (with 5 wt% NaOH solution for 30 min) on tensile properties of flax, linen and bamboo single-strand yarns, surface morphology and mechanical properties (with respect to tensile and flexural properties) of the composites were investigated. It was found that the failure mechanism of single-strand fibres under tension consists of fibre breakage and slippage simultaneously. The alkali treatment had a negative effect on the tensile strength and modulus of the flax, linen and bamboo single-strand yarns. However, after the treatment, the tensile and flexural properties of all the composites increased, e.g. the tensile and flexural strength of the treated flax/epoxy composite increased 21.9% and 16.1%, compared to the untreated one. After the treatment in all the composites, the tensile fractured surfaces exhibited an improvement of fibre/epoxy interfacial adhesion.