Technical Abstract: Microfluidized and stearic acid modified soy protein aggregates were used to reinforced natural rubber. The size of soy protein particles was reduced with a microfluidizing and ball milling process. Filler size reduction with longer ball milling time tends to increase 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. Increasing the extent of modification with stearic acid from 5 to 10% did not improve 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.