In this article, graft copolymerization of styrene (St) onto cassava starch via potassium diperiodatonickelate (IV)–starch redox system as an initiator was investigated in an alkaline medium. The effects of reaction variables were investigated, and the grafting conditions were optimized. The equation of the polymerization rate were obtained, Rp= k•C1.02 (St)•C0.53 (Ni（IV）). The graft copolymer was characterized with Fourier-transform infrared spectra analysis (FTIR) and scanning electron microscopy (SEM). FTIR spectra indicated the presence of PSt-g-starch copolymer. SEM micrographs also showed PSt adhering on the starch.