In this study, a new biodegradable co-polyester was prepared and characterized based on its crystallinity and thermal properties. The crystallinity decreased as the content of long chain monomers, such as adipic acid, in the main chain of poly(butylene succinate) increased, whereas the flexibility increased. Compared with an immiscible blended resin having the same monomer composition, the flexibility and elongation of the synthesized co-polyester was superior without compromise of biodegradability.