The effects of fumed silica on the crystallization behavior and thermal properties of poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) were investigated. The PHBV/silica composites were prepared by a melt-blending method. The nonisothermal crystallization, melting process, and isothermal crystallization kinetics of PHBV and PHBV/silica composites were characterized with differential scanning calorimetry. The spherulite development and morphology were observed by polarized optical microscopy. In addition, the thermal degradation properties were determined via thermogravimetric analysis. The results indicated that the melting and crystallization kinetics of PHBV were greatly affected by fumed silica, and this was due to the effective nucleation function of silica, which enhanced the crystallization process. The thermal onset degradation temperature of PHBV increased with the addition of fumed silica. (c) 2008 Wiley Periodicals, Inc.