The tensile strength and Young’s modulus of sisal fibre bundles were determined following alkalisation. The results were then analysed with respect to the diameter and internal structure such as cellulose content, crystallinity index and micro-fibril angle. The tensile strength and stiffness were found to vary with varying concentration of caustic soda, which also had a varying effect on the cell wall morphological structure such as the primary wall and secondary wall. The optimum tensile strength and Young’s modulus were obtained at 0.16% NaOH by weight. The stiffness of the sisal fibre bundles obtained using the cellulose content also referred to as the micro-fibril content was compared with the stiffness determined using the crystallinity index. The stiffness obtained using the crystallinity index was found to be higher than that obtained using the cellulose content however, the difference was insignificant. Alkalisation was found to change the internal structure of sisal fibres that exhibited specific stiffness that was approximately the same as that of steel. These results indicates that the structure of sisal fibre can be chemically modified to attain properties that will make the fibre useful as a replacement for synthetic fibres where high stiffness requirement is not a pre-requisite and that it can be used as a reinforcement for the manufacture of composite materials.