This thesis studies both the short-term and long- term behaviour of sisal and coconut fibre reinforced mortar composites.The experimental work involved extensive laboratory testing to determine the physical and mechanical properties of the fibre reinforcement and to study the influence of fibre type, volume fraction, fibre length, fibre arrangement and matrix composition on the mechanical properties of the composite.Studies were also made to determine the influence of fibre reinforcement in controlling free and restrained shrinkage during the early age of mortar mixes. Cracking due to restraint and the phenomenon of crack self-healing were also investigated. The mode of failure and the properties of the resistance to fibre-matrix interfacial bonding were determined using the single fibre pull-out test.The long-term properties of the sisal and coconut fibre reinforced-mortar composites were assessed throughout creep, shrinkage and durability tests. The influence of the addition of sisal and coconut fibres, of various volume fraction and lengths, on the creep of a mortar matrix was determined using sealed and unsealed specimens subjected to a pressure of 14.4 MPa over a period of 210-350 days. Recovery strains were recorded for a period of 56-180 days.The influence of fibre types, volume fraction, fibre lengths, cure types, mix proportions and replacement of OPC by slag and silica fume on the dimensional stability of mortar matrices was determined using drying shrinkage tests for a period of 320 days. The durability of sisal and coconut fibres exposed to alkaline solutions of calcium and sodium hydroxide and stored in tap water was measured as strength loss over a period of 420 days. The durability of fibre-reinforced mortars after 320 to 360 days, stored under water, exposed to cycles of wetting and drying as well as to the natural weather,was assessed from results of flexural tests and from observations of the photomicrographs obtained using backscattered imaging and secondary electron imaging. Dotting maps of chemical elements were obtained in order to verify possible migration of cement products from the matrix to the lumen and voids within of the fibres. Treatments to enhance the durability performance of the composites were studied,including: (a) modifications to the matrix through the replacement of Portland cement by undensified silica fume and by blast-furnace slag; (b) carbonation of the cementitious matrix and (c) immersion of the fibres in slurry silica fume prior to being incorporated into the Portland cement matrix.