Poly(N-vinylpyrrolidone) (PVP) groups were grafted onto poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) backbone to modify the properties of PHBV and synthesize a new novel biocompatible graft copolymer. The effect of graft modification with PVP on the thermal and mechanical properties of PHBV was investigated. The thermal stability of grafted PHBV was remarkably improved while the melting temperature (T-m) was almost not affected by graft modification. The isothermal crystallization behavior of samples was observed by polarized optical microscopy and the results showed that the spherulitic radial growth rates (G) of grafted PHBV at the same crystallization temperature (T-c) decreased with increasing graft yield (graft%) of samples. Analysis of isothermal crystallization kinetics showed that both the surface free energy (sigma(e)) and the work of chain-folding per molecular fold (q) of grafted PHBV increased with increasing graft%, implying that the chains of grafted PHBV are less flexible than un-grafted PHBV. This conclusion was in agreement with the mechanical testing results. The Young's modulus of grafted PHBV increased while the elongation decreased with increasing graft%. The hydrophilicity of polymer films was also investigated by the water contact angle measurement and the results revealed that the hydrophilicity of grafted PHBV was enhanced. (C) 2008 Wiley Periodicals, Inc.