In this paper, three kinds of unidirectional sisal/epoxy composites are studied. The materials is made of untreated sisal, alkali treated sisal and sisal exposed in moisture. The tensile strength and modulus, work of fracture have been measured as a function of fiber volume fraction from 30% to 70%. These properties all follow a Rule of Mixtures relationship with Vf. Because the alkali treatment modifies the interfibrillar region of sisal, the sisal fiber become easier wetted by epoxy resin. As a result, th...