Technical Abstract: Dynamic mechanical properties including temperature effect, stress softening, and Payne effect are studied on the elastomer composites filled with soy protein or carbon black. The comparison of protein composite with well-known carbon black composites provides further insight into the protein composites. The Elastomers filled with soy protein aggregates give a substantial reinforcement effect when compared with the unfilled elastomers. Approximately 400 times the increase in shear elastic modulus was observed when 40% by weight of protein is incorporated into the elastomers. The sample films were cast from the particle dispersion of soy protein isolate and carboxylated styrene-butadiene latex. At the higher temperatures, the shear elastic modulus of soy protein filled composites does not decrease as much as that of the carbon black filled composites. The behavior of elastic and loss modulus under the oscillatory strain of different magnitudes is similar to that of carbon black reinforced styrene-butadiene rubber. However, carbon black composites show a better recovery behavior after eight cycles of dynamic strain. The reduction of shear elastic modulus with dynamic strain (Payne effect) was compared with Kraus model and the fitting parameter related to the aggregate structure of the soy protein. A reasonable agreement between the theoretical model and experiment was obtained, indicating the Payne effect of the protein related network structure in the elastomers could also be described by the kinetic agglomeration de-agglomeration mechanism.