A series of novel biodegradable poly(ester‐carbonate) consisting of poly(L‐lactic acid) (PLLA) and poly(butylene carbonate) (PBC) (PLLA‐b‐PBC) have been successfully synthesized by chain‐extension reaction of dihydroxyl‐terminated poly(L‐lactic acid) prepolymer (PLLA‐OH) and dihydroxyl‐terminated poly(butylene carbonate) prepolymer (PBC‐OH) using hexamethylene diisocyanate as a chain‐extender. The chemical structures and molecular weights of copolymers are characterized by 1H NMR and gel permeation chromatography. The thermal properties, morphology, and mechanical properties of PLLA‐b‐PBC are investigated by differential scanning calorimetry, thermogravimetric analysis, phase contrast optical microscopy, and mechanical testing. The results indicate that PBC soft segment slightly increases the thermal stabilities and effectively enhances the impact strength of the copolymers. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014, 131, 39158.