Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV)/poly(3-hydroxybutyrat e-co-4-hydroxybutyrate) (P3/4HB) blend films were prepared by solvent-cast method. The nonisothermal crystallization results showed that PHBV and P3/4HB are miscible due to a single glass transition temperature (T-g), which is dependent on blend composition. The isothermal crystallization results demonstrate that the crystallization rate of PHBV becomes slower after adding amorphous P3/4HB with 19.2 mol%, 4HB, which could be proved through depression of equilibrium melt point (T-m(o)) from 183.7 degrees C to 177.6 degrees C. For pure PHBV and PHBV/P3/4HB (80/20) blend, the maximum crystalli-zation rate appeared at 88 degrees C and 84 degrees C, respectively. FTIR analysis showed that PHBV/P3/4HB blend films would maintain the helical structure, similar to pure PHBV. Meanwhile, with increasing P3/4HB content, the inter- and intra-interactions of PHBV and P3/4HB decrease gradually. Besides, a lower elastic modulus and a higher elongation at break were obtained, which show that the addition of P3/4HB would make the brittle PHBV to ductile materials. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 838-848, 2010