The soy protein isolate (SPI)/Na+-montmorillonite (MMT) plastics were successfully prepared, and their structures and properties were characterized with X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, thermogravimetric analysis, and tensile testing. The interactions between the soy protein macromolecules and MMT in aqueous media were analyzed with xi-potential measurements, Fourier transform infrared spectroscopy, and electrostatic surface potential calculations. The results revealed that the heterogeneous distribution of the surface positive charges provided the positive-charge-rich domains for the soy globulins bearing net negative charges to anchor into the negatively charged MMT galleries. There were electrostatic attraction and hydrogen bonding interactions on the interfaces of the soy protein and MMT, which led to the good dispersion of the phyllosilicate layers in the protein matrix. The highly exfoliated MMT layers with a dimension of 1-2 nm in thickness were randomly dispersed in the protein matrix containing MMT lower than 12 wt %, whereas the intercalated structure was predominant when the MMT content was higher than 12 wt %. Consequently, the fine dispersion of the MMT layers and the strong interactions between SPI and MMT created the significant improvement of the mechanical strength and thermo-stability of the SPI/MMT plastics. In addition, a schematic illustration was proposed to describe the electrostatic interaction between SPI and MMT as well as the correlation between the interaction and structure in protein/clay systems.