Technical Abstract: The use of renewable soy protein aggregates as a reinforcement network in Styrene-butadiene rubber composites is investigated by a dynamic mechanical method. The rigid nature of dry protein has a high shear elastic modulus of ~2 GPa and, therefore, is suitable as a reinforcement phase in rubber composites. The addition of soy protein to the rubber composites generates a significant reinforcement effect. The characterization includes the dynamic mechanical and FTIR studies of the annealing effect on the composite modulus. The increasing elastic modulus with time in the constant temperature experiments indicates the hardening of protein through a mechanism of dehydration and structure change. This is also accompanied by an increase in protein density. The apparent rate of modulus increase during annealing at different elevated temperatures up to 140oC does not show a significant difference. A comparison is also made with rubber composites prepared from an aqueous dispersion of carbon black. The result indicates dry protein composites have a higher shear elastic modulus at the same weight fraction of filler.