Technical Abstract: Soy protein nanoparticle aggregates were prepared by alkaline hydrolysis of soy protein isolate (SPI). Light scattering measurements indicated a narrow size distribution of SPI aggregates. Nanocomposites were formed by mixing hydrolyzed SPI (HSPI) nanoparticle aggregates with styrene-butadiene (SB) latex. At 140 deg C, the composites filled with 30% HSPI exhibited roughly a 540-fold and 9-fold increase in G’ compared to the unfilled SB rubber for the composites prepared at pH 9 and 5.2, respectively. Compared to SPI, the glass transition temperatures and the broadening effect of G” maxima indicated that HSPI has a stronger filler-polymer interaction and is more homogeneous in its polymer immobilization effect. Strain sweep and recovery experiments indicated HSPI filled composites had better modulus retention than SPI composites. Composites prepared at pH 5.2 had better modulus retention than those prepared at pH 9 with the exception of the 30% filled SPI composites. The model fitting of the reversible strain sweep data indicated that HSPI composites are more elastic, while SPI composites have a higher shear modulus. The model fitting also indicates the composites prepared at pH 5.2 are more elastic, but the composites prepared at pH 9 have a higher modulus. Fractal dimensions estimated from the strain sweep and temperature sweep experiments are in a good agreement and indicate HSPI has a more compact aggregate structure than SPI aggregates.