Multiblock copolymers consisting of poly(l-lactic acid) and poly(dimethyl siloxane) were prepared by the polycondensation of oligo(l-lactic acid) (OLLA) with dihydroxyl-terminated oligo(dimethyl siloxane) and dicarboxyl-terminated oligo(dimethyl siloxane). Copolymers with number-average molecular weights of 18,000−33,000 Da and various content ratios of oligo(dimethyl siloxane) (ODMS) unit were obtained by changing the feed ratio of these oligomers. A film prepared from the copolymer with an ODMS content ratio of 0.37 exhibited two independent peaks at −107°C and 37°C in the mechanical loss tangent for temperature dependence, suggesting the formation of microphase separation between the OLLA and ODMS segments. The film had a tensile strength of 3.2 MPa and a high elongation of 132%. The film also exhibited a high strain recovery even after repeated straining. The incorporation of dimethyl siloxane units as multiblock segments was confirmed to improve the flexibility of poly(l-lactic acid). © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40211.