The tensile properties of polystyrene reinforced with short sisal fiber and benzoylated sisal fiber were studied. The influence of fiber length, fiber content, fiber orientation, and benzoylation of the fiber on the tensile properties of the composite were evaluated. The benzoylation of the fiber improves the adhesion of the fiber to the polystyrene matrix. The benzoylated fiber was analyzed by IR spectroscopy. Experimental results indicate a better compatibility between benzoylated fiber and polystyrene. The benzoylation of the sisal fiber was found to enhance the tensile properties of the resulting composite. The tensile properties of unidirectionally aligned composites show a gradual increase with fiber content and a leveling off beyond 20% fiber loading. The properties were found to be almost independent of fiber length although the ultimate tensile strength shows marginal improvement at 10 mm fiber length. The thermal properties of the composites were analyzed by differential scanning calorimetry. Scanning electron microscopy was used to investigate the fiber surface, fiber pullout, and fiber-matrix interface. Theoretical models have been used to fit the experimental mechanical data.