An A-B-A block copoly(ester-ether) comprising poly(L-lactic acid)(PLLA, A) and poly (oxyethylene)(POE, B) was prepared by copolymerization. The release rate of the model drugs form PLLA/POE particles was studied by varying the hydrophilicity of the model drugs as well as that of the copolymers. It was observed that the rate of controlled release from the copolymer microspheres is not dependent on the degradation of the polymer, but on the wall permeability which is defined as the product of diffusion coefficient and the solubility coefficient. The release rate was fast as long as the hydrophilicity of the copolymer was high, regardless of the composition of the copolymer. It was also found that the release rate was faster as the monomer composition of the oxyethylene unit becomes higher.