A polypropylene (PP)-grafted polyhydroxybutyrate (PHB) copolymer was synthesized by a three-step procedure. The existence of the PP-g-PHB copolymer was confirmed by IRFT analysis. Blends at different ratios of PP and PHB, in the presence and in absence of the grafted copolymer, were prepared. The mechanical properties of the blends were determined by tensile test and solid-state spectroscopy. Morphology studies were also conducted using scanning electron microscopy. The presence of only 5% of the copolymer in a PP-PHB blend affects dramatically the morphology as the domains of PHB seem to vanish into the PP matrix. The copolymer affects favorably the yield stress but affects negatively the properties at break. The calculated rheological properties using Dickie's model are compared with the experimental values.