Biodegradable polymer blends prepared by blending poly(3-hydroxybutyrate) (PHB) and corn starch do not form intact films due to their incompatibility and brittle behavior. For improving their compatibility and flexibility, poly(vinyl acetate) (PVAc) was grafted from the corn starch to prepare the PVAc-modified corn starch (CSV). The resulting CSV consisted of 47.2 wt% starch-g-PVAc copolymer and 52.8 wt% PVAc homopolymer and its structure was verified by FT-IR analysis. In comparison with 35°C of the neat PVAc, the glass transition temperature (Tg) of the grafted PVAc chains on starch-g-PVAc was higher at 44°C because of the hindered molecular mobility imposed from starch on the grafted PVAc. After blending PHB with the CSV, structure and thermal properties of the blends were investigated. Only a single Tg was found for all the PHB/CSV blends and increased with increasing the CSV content. The Tg-composition dependence of the PHB/CSV blends was well-fitted with the Gordon-Taylor equation, indicating that the CSV was compatible with the PHB. In addition, the presence of the CSV could raise the thermal stability of the PHB component. It was also found that the presence of the PHB and PVAc components would not hinder the enzymatic degradation of the corn starch by α-amylase.