Technical Abstract: Viscoelastic properties of natural rubber (NR) composites reinforced by defatted soy flour (DSF) are investigated. DSF is an abundant renewable commodity and has a lower material cost than carbon black (CB). DSF contains soy protein, soy carbohydrate, and soy whey. Aqueous dispersion of DSF was blended with natural rubber latex to form rubber composites by freeze-drying and compression molding method. A 40% DSF reinforced composite exhibited about a 60-fold increase in the shear elastic modulus compared with unfilled natural rubber, showing a significant reinforcement effect by DSF. The effect, however, is lower than the carboxylated styrene-butadiene rubber composites reported earlier, indicating a significant effect of rubber matrix. Mixtures of DSF and CB at three different ratios were also investigated as co-fillers. The co-filler composites have elastic moduli that were between those of DSF and CB reinforced composites. The recovery experiments indicated the co-filler composites with a higher CB content have a better recovery behavior, which however can not be predicted from the recovery behavior of the single filler (DFS and CB) composites.