Thermal properties of blends of poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) and poly(styrene-co-acrylonitrile) (SAN) prepared by solution casting were investigated by differential scanning calorimetry. In the study of PHBV-SAN blends by differential scanning calorimetry, glass transition temperature and melting point of PHBV in the PHBV-SAN blends were almost unchanged compared with those of the pure PHBV. This result indicates that the blends of PHBV and SAN are immiscible. However, crystallization temperature of the PHBV in the blends decreased approximately 9 -15Β°. From the results of the Avrami analysis of PHBV in the PHBV-SAN blends, crystallization rate constant of PHBV in the PHBV-SAN blends decreased compared with that of the pure PHBV. From the above results, it is suggested that the nucleation of PHBV in the blends is suppressed by the addition of SAN. From the measured crystallization half time and degree of supercooling, interfacial free energy for the formation of heterogeneous nuclei of PHBV in the PHBV-SAN blends was calculated and found to be 2360 (mN/m) 3 for the pure PHBV and 2920 -3120 (mN/m) 3 for the blends. The values of interfacial free energy indicate that heterogeneity of PHBV in the PHBV-SAN blends is deactivated by the SAN. This result is consistent with the results of crystallization temperature and crystallization rate constant of PHBV in the PHBV-SAN blends.