Atactic poly(3-hydroxybutyrate) (a-PHB) and block copolymers of poly(ethylene glycol) (PEG) with poly(epsilon -caprolactone) (PCL-b-PEG) were synthesized through anionic polymerization and coordination polymerization, respectively. As demonstrated by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA) measurements, both chemosynthesized a-PHB and biosynthesized isotactic PHB (i-PHB) are miscible with the PEG segment phase of PCL-b-PEGs. However, there is no evidence showing miscibility between both PHBs and the PCL segment phase of the copolymer even though PCL has been block-copolymerized with PEG. Based on these results, PCL-b-PEG was added, as a compatibilizer, to both the PCL/a-PHB blends and the PCL i-PHB blends. The blend films were obtained through the evaporation of chloroform solutions of mixed components. Excitingly, the improvement in mechanical properties of PCL/PHB blends was achieved as anticipated initially upon the addition of PCL-b-PEG.