Abstract: This paper investigated the evolution of mechanical properties of flax fibre reinforced polymer compo-sites (FFRPs) conditioned in humid condition for a long term. Dry unidirectional FFRPs having fibre volume fraction of 40vol% were manufactured via vacuum assisted resin transfer moulding (VARTM). FFRPs were conditioned in 30°C, 80% relative humidity (RH) for different time (5 days, 35 days and 86 days) and their tensile properties were tested and analyzed. The results demonstrate that the moisture absorption of FFRPs fairly follows one dimensional Fickian’s second law. The equilibrated water content is around 5.3%. Tensile strength and modulus perpendicular to fibre direction decrease with moisture absorption. Fracture morphology shows that fibre-matrix bonding strength decreases after moisture absorption. Tensile strength in fibre direction is not degraded by moisture absorption, and exhibits a trend featured by a first drop followed by an increase, which has not been reported in literatures. Tensile strength in fiber direction decreases by 5.7% after being conditioned in humid for 5 days, and increases by 18.7% after 35 days. Further absorption of moisture up to 86 days (already saturated) causes a slight decrease in tensile strength but is still 13.7% higher than that at dry state. The change trend of tensile strength in fibre direction during moisture absorption can be explained as a consequence of averaging effects of several factors.