This research used the ceric ion to initiate the graft-polymerization of vinyl acetate (VAc) to a soluble potato starch. Fourier transform infrared spectra confirmed the formation of starch graft copolymer. After 4 h of reaction at 50 degrees C, total monomer conversion, grafting efficiency, and grafting ratio were measured as 91%, 12.5%, and 0.223, respectively. The synthesized PVAc-modified starch was then blended with poly(3-hydroxybutyrate) (PHB). Structures, thermal and mechanical properties of the prepared blends were examined. The results showed the PHB and PVAc-modified starch were miscible in all compositions. In addition, thermal gravimetric analysis revealed that the addition of PVAc-modified starch increased the thermal stability of the PHB component. Further evidence also showed that the addition of PVAc-modified starch reduced the extent of decrease in molecular weight of PHB in a melt-mixer. PHB/PVAc-modified starch blends exhibit higher toughness than pure PHB because of increased compatibility and the leathery PVAc-modified starch. POLYM. ENG. SCI., 50:709-718, 2010. (C) 2009 Society of Plastics Engineers