The aim of this study was to determine the effect of moisture cycling (environmental relative humidity cycles) on the durability of flax-epoxy composites and investigate the influence of cycling duration and temperature on the stiffening and strengthening of flax fibres. Four moisture cycling protocols for flax fibres were employed in this research which includes 4D21 (4days per cycle at 21ºC), 4D60 (4days per cycle at 60ºC), 3H27 (3hours per cycle at 27ºC) and 3H60 (3hours per cycle at 60ºC). To measure the impact of high-low humidity cycling at different cycling durations and temperature, tensile testing of impregnated fibre bundle test (IFBT) samples was done. Results of the back-calculated properties revealed that the applied cycling protocols enhanced both the tensile strength and modulus of the fibres. Better improvement of tensile properties was observed in fibres cycled at longer duration. The fibres undergoing 4 days of cycling at 21ºC (4D21 fibres) showed the highest improvements in tensile strength (18%), as well as tensile moduli E1 (19%) and E2 (18%) after 10 cycles. Interestingly, all fibres showed increased stiffness (E1) in the range of 8-20% after 10 cycles and 4-8% after 20 cycles. This fibreimprovement in mechanical strength and stiffness of the fibres can possibly be attributed to a phenomenon similar to a hornification effect in wood or possibly by fibre repair due to pectin migration, which produces the strengthening and stiffening effect.@en