A series of soy protein/kaolin nanocomposites have been successfully prepared via solution mixing and then extrusion. The structure and the properties of the nanocomposite plastics were characterized by wide-angle X-ray diffraction (WAXD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), water uptake as well as mechanical testing. WAXD revealed that the nano-sized kaolin filler in soy protein matrix retained the original structure as the pristine kaolin. In view of the results of SEM, kaolin distributed in the protein matrix homogeneously and exhibited fine adhesion with soy protein. The incorporation of kaolin into glycerol-plasticized soy protein led to great improvements of mechanical and water resistant properties. The thermal stability was also enhanced by the incorporation of kaolin as revealed by TGA characterization.