Poly(3-hydroxybutyrate-co-hydroxyvalerate) (PHBV) and poly(ethylene succinate) (PES) are both biodegradable semi-crystalline polyesters. Phase behavior, crystal structure, crystallization kinetics, and mechanical properties of fully biodegradable PHBV/PES blends were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), wide angle X-ray diffraction (WAXD), and tensile tests in detail. PHBV is immiscible with PES as evidenced by the two composition independent glass transition temperatures corresponding to those of neat components. Moreover, the biphasic melt further confirms the immiscibility between PHBV and PES. Nonisothermal and isothermal crystallization of PHBV/PES blends were studied by DSC. In the case of isothermal crystallization the crystallization rates of PHBV and PES decrease with increasing the content of the other component for both PHBV-rich and PES-rich blends; however, the crystallization mechanisms do not change. The mechanical properties of PHBV/PES blends were examined by tensile testing. It is found that the tensile strength, elongation at break, Young's modulus, and toughness of PHBV blends can be adjusted by changing blend composition. For PHBV-rich blends, blending some amount of PES can improve the elongation at break significantly while still keeping considerably high tensile strength and Young's modulus. (C) 2007 Elsevier Ltd. All rights reserved.