The nanO-SiO2 particles were compounded into soy protein isolated (SPI) matrix to produce a series of reinforcing nanocomposite sheets by compression-molding. Except for the expected increase of strength and modulus, the elongation was also enhanced when the nanO-SiO2 content was lower than 8 wt %. Moreover, two nanocomposite materials were recommended: the one is a nanocomposite containing 4 wt % nano-SiO2 with the highest strength and enhanced elongation, the other is a reinforced material with the best elongation filled by 8 wt 0% nano-SiO2. The increase of nano-SiO2 content produced many kinds of distributions in SPI matrix, such as single nanosphere, - 100 nm nanocluster, interconnected network structure and great domain. Such structures strongly affected the mechanical performances of nanocomposite materials. The simultaneous enhancement of strength and elongation was related to homogeneous dispersion of nanoclusters while aggregated great domains severely decreased elongation in spite of obvious reinforcing effect. However, the reinforced materials with high loading of inorganic filler should be paid attention and have economic value to some extent in practical application. With the changes of nano-SiO2 distribution, the structures of SPI matrix changed as well. After adding a mall amount of nano-SiO2, the damage of glycerol plasticization to ordered structure of SPI was reduced. But as nano-SiO2 content increased, the SPI microphase was separated from nano-SiO2 domains. Furthermore, the condition of simultaneous reinforcing and toughening was put forward: the moderate aggregation of nano-SiO2 as well as all kinds of strong interfacial interactions. (c) 2007 Wiley Periodicals, Inc.