Natural rubber was enhanced with soy protein nano-aggregates and carbon black using a hybrid process. The rubber composites reinforced with an optimum amount of soy protein or soy protein/carbon black showed useful tensile properties. The stress-strain behaviors were analyzed with a micro-mechanical model that describes the stress-strain measurements well. The model analysis provides insight into filler network characteristics and entanglement modulus. The composites were also analyzed with both linear and nonlinear viscoelastic properties. Temperature and frequency dependent modulus as well as the model analysis of stress softening effect describe the ability of soy protein to constraint polymer chains in the highly filled composites. For the composites reinforced with soy protein, the good tensile properties are attributed to good filler-polymer adhesion through the compatibilization effect of natural rubber protein. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2188-2197, 2013