Three series of poly(ethylene glycol)methyl ether (MPEG)/poly(lactic acid) (PLA) block copolymer systems, composed of MPEG in the molecular weights of 2000, 5000, and 750 g/gmol, respectively, and PLA in different molecular weights were synthesized. The chemical structure and physical properties were investigated using FTIR, NMR, DSC, TGA, and GPC. Different crystallization and melting transition behavior were observed according to the variation of the component block length. Either nano-sized micelles or solid particles were formed in water, depending on the component block length and its ratio. The mean diameter of spherical drug carriers was controlled by the preparation conditions and was characterized using DLS and SEM. Indomethacin was loaded into nano-sized spherical polymer carriers. The releasing rates were significantly affected by not only the sizes but also by the physical state of the drug carriers in the media, resulting from the difference in the component block length and the molecular entanglement of the PLA cores.