A computer model is presented for the study of the effects of fiber characteristics on the mechanical properties of short-fiber-reinforced composites. Our model results indicate a very weak effect of fiber orientation on the modulus and tensile strength of the composite. Very high levels of orientation are required before a true reinforcement of the composite by the fiber can occur. For randomly dispersed fibers, our results point to the presence of an optimum fiber aspect ratio for effective reinforcement. They also indicate a microfailure mechanism which originates at the fiber ends and propagates along the fiber-matrix interface, with no fiber breaking being involved. Our model results are compared with predictions from other theories and available experimental data.