Poly(ethylene glycol)-poly(L-lactide) diblock and triblock copolymers were prepared by ring-opening polymerization of L-lactide with poly(ethylene glycol) methyl ether or with poly(ethylene glycol) in the presence of stannous octoate. Molecular weight, thermal properties, and crystalline structure of block copolymers were analyzed by H-1-NMR, FTIR, GPC, DSC, and wide-angle X-ray diffraction (WAXD). The composition of the block copolymer was found to be comparable to those of the reactants. Each block of the PEG-PLLA copolymer was phase separated at room temperature, as determined by DSC and WAXD. For the asymmetric block copolymers, the crystallization of one block influenced much the crystalline structure of the other block that was chemically connected to it. Time-resolved WAXD analyses also showed the crystallization of the PLLA block became retarded due to the presence of the PEG block. According to the biodegradability test using the activated sludge, PEG-PLLA block copolymer degraded much faster than PLLA homopolymers of the same molecular weight.