New thermoplastic soy protein isolated (SPI) and methyl methacrylate (MMA) copolymers (T-SPI) were prepared using graft copolymerization and initiated with ammonium persulfate (APS). The reaction conditions such as initiator concentration and temperature on the graft percentage (GP) were investigated. The single factor experimental showed that the optimal conditions of the graft reaction. The products were characterized by means of FT-IR and DSC. The results indicated that the monomer was grafted successfully on SPI and the T-SPI had a glass transition at 122°C, and the most important, the water absorption of materials declined obviously.