Miscibility and mechanical properties of a ternary blend of poly(3-hydroxybutyrate) (P3HB)/poly(L-lactide) (PLLA)/cellulose esters (CEs) were investigated by X-ray diffraction, and dynamical mechanical analysis. The binary blend of P3HB and PLLA is generally immiscible if high molecular weight PLLA is used; however when only a small amount of 5 wt% cellulose esters such as cellulose acetate butyrate (CAB), cellulose acetate propionate (CAP), and cellulose propionate (CP) were added to the P3HB/PLLA blend, the crystallization of P3HB was significantly inhibited. Single glass transition temperature of P3HB/PLLA blends with 5 wt% cellulose esters was observed and can be fitted by Fox’s equation extended to the ternary blend. This indicates that P3HB and PLLA are miscible when only 5 wt% cellulose ester, which acts as a compatibilizer, is added. The mechanical properties such as tensile modulus and elongation at break of (4/6) and (3/7) P3HB/PLLA blends with 5 wt% CP were almost identical to PLLA; however the maximum stress of these blend films was improved in comparison with the homopolymer. When 50 wt% CP was mixed to (7/3) P3HB/PLLA blend, the elongation at break became 77.2%. Since CP inhibits the crystallization of P3HB, it can be assumed that this blend film shows ductility.