Due to increased awareness of environmental issues, considerable attention has recently been given to the development of natural fibre reinforced polymer composites. Flax fibres and other natural fibres have various advantages; such as low cost, abundance, renewability and can offer respectable mechanical properties. However, flax fibre is a hygroscopic material and concern has been raised for composites manufacturing at high fibre moisture content (MC) which could have a negative effect on the mechanical properties of flax fibre composites. In this study, the relationship between weight percentage (wt.%) fibre MC and the mechanical performance of flax/epoxy composites was investigated. Using dynamic vapour sorption (DVS) analysis, an increase in fibre MC with increasing ambient relative humidity (RH) was observed. The flax fibre absorbed as high as 14.8 wt.% MC. A novel method of estimating MC was developed using a standardized flax fabric sample which was useful for monitoring fibre MC during composites manufacturing. The fibre-matrix interfacial shear strength (IFSS) was evaluated using the microbond technique and IFSS was found to reduce consistently with increasing fibre MC for untreated-fibre composites. Tensile, flexural and low velocity impact tests were conducted on the flax/epoxy composites, and in general properties reduced with increasing fibre MC. Scanning electron microscopy (SEM) also indicated weak fibre-matrix interface of composites as shown by fibre pull-out and the formation of gaps between the fibres and the matrix. Dynamic mechanical analysis (DMA) showed high fibre MC plasticised the epoxy matrix, with values of glass transition temperature reducing from 162.0 ÃÂÃÂºC to 156.6 ÃÂÃÂºC. The measured propagation Mode-I interlaminar fracture energy, GIC propagation, increased from 1.81 kJ/m2 (at 2.3 wt.% fibre MC) to 2.48 kJ/m2 (at 6.6 wt.% fibre MC) due to toughening of the matrix with increasing MC, resulting in a larger plastic zone at the crack tip. A mechanism of moisture interaction at the fibre-matrix interface during untreated flax/epoxy composites manufacturing is proposed, suggesting weaker fibre-matrix interface resulted from disruption of crosslink formation between reactive groups on the surface of the moisturized fibre and the epoxy matrix. Alkaline and silane fibre surface treatments, and combinations of both, were found to reduce fibre moisture absorption between 10-70% RH. Microbond tests showed that IFSS was generally improved after surface treatments with 3.0% NaOH giving the highest values. In most cases, the tensile and flexural properties of treated composites were lower than untreated composites due to loss of tensile properties of flax fibre after being treated, which the weakened internal structure of fibres as a result of swelling. Mode I GIC propagation of treated composites was significantly improved compared to untreated composites at low MC due to a stronger fibre matrix interface.