The mechanical properties of natural rubber composites filled with cellulose short fibers were investigated with respect to fiber concentration and fiber orientation. Strong anisotropy caused by the fiber orientation was observed in the mechanical properties at high fiber loading. The ultimate tensile strength in the longitudinal direction to the fiber orientation had a minimum atVf=12.5%, while the transverse strength decreased withVfvalues. The logarithmic elongation at break decreased linearly withVf, and the transverse elongation was higher than the longitudinal elongation. The modulus of the composite increased with fiber concentration, with semiempirical equationE=k/{1+[Vm/(1−Vm0.5)]} being valid for the transverse direction and the longitudinal direction at lower fiber loading. The longitudinal modulus was much higher than the transverse modulus. In order to improve adhesion between the fibers and matrix, polyallylacrylate was grafted on cellulose fibers, but grafting was ineffective on the mechanical properties of the composite. The dynamic viscoelastic properties of the natural rubber composite were also investigated. The dynamic modulus,E’, increased with increasing amounts of fiber concentration above the glass transition temperature. The loss modulus,E‘, had a peak at about −36 °C; which shifted to higher temperatures with increasing amounts of mixed fibers. The activation energy for glass transition of the composites was calculated at 198.6–228.6 kJ/mol, depending on fiber concentration for the longitudinal direction. The dynamic viscoelasticity in the transverse direction to the fiber orientation was not affected by filled fibers and the activation energy was 187.3 kJ/mol, approximately the same value as for the unfilled rubber vulcanizate.