Natural rubber was reinforced with stearic acid-modified soy protein particles prepared using a microfluidizing and ball milling process. Longer ball milling time tends to increase the tensile strength of the rubber composites. Elastic modulus of the composites increased with the increasing filler concentration. The loss modulus and loss tangent indicated an increase in the amount of polymer immobilized by the modified soy protein. The extent of stearic acid modification affected the mechanical properties of the rubber composites. Compared with the unmodified soy protein, the microfluidized and stearic acid-modified soy protein filler improved the mechanical properties of the rubber composites.