A novel biodegradable copolymer, poly(1,4-dioxan-2-one) (PPDO) grafted soy protein isolate (SPI) (SPI-g-PPDO), was investigated. The ring-opening graft copolymerization of SPI-g-PPDO was carried out with stannous octoate as a co-initiator/catalyst in the presence of azeotropic solvent at 80 degrees C. The molecular structure of SPI-g-PPDO was characterized by Fourier transform infrared (FTIR) spectroscopy and nuclear magnetic resonance (NMR). The results suggested that the content and length of PPDO graft chains of the copolymer increased with the increase of monomer (1,4-dioxan-2-one, PDO) content. Moreover, the results of differential scanning calorimetry (DSC) showed that the copolymer has a definite glass transition temperature and melting point, meaning that the copolymer can be melt-processed. In addition, the copolymers could form crystals after the grafting polymerization. The crystallization ability increases with the increase of graft chain length.