This paper reports the use of a natural fibre, isora, as reinforcement in unsaturated polyester resin. Isora is a bast fibre separated from the bark of Helicteres isora plant by retting process. Properties like tensile strength, flexural strength etc. have been studied as a function of fibre length and fibre loading using treated and untreated fibre. The mechanical properties were found to be optimum at a fibre length of 30 mm and a fibre loading of 30% by volume. The effects of alkali treatment on the fibre properties were investigated by SEM, IR and TGA. The mechanical performance of the treated isora fibre-reinforced polyester composites has also been investigated. SEM studies were carried out to investigate the fibre surface morphology, fibre pull-out and fibre-polyester interface bonding. SEM gave evidence for the changes that had occurred on the fibre surface during chemical treatment. The properties were found to be superior for the composite reinforced with treated fibre compared to the untreated fibre.