The graft copolymer, poly[(hydroxybutyrate-co-hydroxyvalerate) vinyl ketone] [CP(HBV-g-PVK)], was synthesized by graft polymerization of phenyl vinyl ketone (PVK) onto poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) under nitrogen atmosphere using benzoyl peroxide. The structure of P(HBV-g-PVK) was identified by Fourier transform TR and H-1-NMR spectra. The effects of weight ratio of PVK to PHBV in feed, initiator concentration, reaction time, and reaction temperature on the grafting ratio and grafting efficiency were investigated. The thermal decomposition temperature of P(HBV-g-PVK) was 272 degrees C. The tensile strengths of P(HBV-g-PVK) after photo- or biodegradation were significantly decreased due to degradation by UV irradiation or Aspergillus niger. The value of color difference (Delta E) of P(HBV-g-PVK) was greater than that of PHBV. The film surfaces of P(HBV-g-PVK) treated with UV irradiation and Aspergillus niger showed many pits as compared with the untreated P(HBV-g-PVK). It has been found that the photo- and biodegradabilities of P(HBV-g-PVK) was excellent.