Triblock copolymers poly(methyl methacrylate)- b -poly(ethylene oxide)- b -poly(methyl methacrylate) (PMMA- b -PEO- b -PMMA) with designed molecular weight of PMMA and PEO blocks were synthesized via the activator regenerated by electron transfer (ARGET) atom transfer radical polymerization (ATRP) of MMA. The Br-terminated Br–PEO–Br with the molecular weights of 20k and 100k were used as  macroinitiators. ARGET ATRP was performed with ppm level amount CuBr 2 as the catalyst and ascorbic acid as the reducing agent to overcome the sensitivity to oxygen in a traditional ATRP. The molecular weight of the PMMA block was manipulated by changing the molar ratio of monomers to the Br–PEO–Br macroinitiators. The synthesis of PMMA- b -PEO- b -PMMA and its structure was confirmed by Fourier transform infrared and 1 H NMR, and the molecular weight of the PMMA block was determined by 1 H NMR. Aqueous solutions of PMMA- b -PEO- b -PMMA were prepared by solvent-exchange, and their microstructures were examined by tensiometry, static light scattering, and transmission electron microscopy. The effects of molecular weight of the PMMA and PEO blocks on the microstructure were elucidated.