Branched poly(lactic acid) was synthesized by ring-opening polymerization of lactide using castor oil as an initiator. The structure of the branched polymer was confirmed by 1 H nuclear magnetic resonance spectroscopy. The molecular weight of the branched polymer increased linearly as a function of the feed ratio of lactide and castor oil. The thermal properties of the branched polymer were evaluated using differential scanning calorimetry and were compared with those of linear poly(L-lactic acid) (PLLA). A blended film of the branched polymer and PLLA was prepared by hot pressing at 175 1C. A content of 5 wt% of the branched polymer was sufficient to increase the strain at break. Thermomechanical analysis of the blended film revealed a decrease in the initial elastic modulus with addition of the branched polymer. The thermal and dynamic mechanical properties of the film were examined, and the crystalline morphology of the film was examined by polarized light microscopy.