the composites of acrylonitrile-butadiene rubber (nbr) and two kinds of short silk fibers (antheraea pernyi and bombyx mori) without surface-treatment were prepared by a bambury mixer. the reinforcing effect of fibers on those composites was investigated by determining mechanical properties. the following results were obtained. under high shear forces during the mixing process, antheraea pernyi fiber easily underwent fibrillation and uniformly dispersed in the matrix of nbr. and so this fibril exerts a reinforcing effect on the composite, resulting in low elongation, high strength, and high modulus of elasticity. on the other hand, bombyx mori fiber did not give rise to fibrillation, but only cut to short fiber lengths, so that these composites did not show any reinforcing effect. therefore, antheraea pernyi fibers have caused an improved specific surface area and surface activity; consequently, interactions between fibrillated antheraea pernyi fiber and matrix rubber have occurred in this composite. (author abstract) 10 refs. in japanese