A series of AB(2) type 3-miktoarm star copolymers that mimic the natural structure of phospolipids were synthesized using poly(ethylene glycol) (PEG) as the A arm and poly(L-lactic acid) (PLLA) as the two B arms. Their ability to self-assemble into polymer vesicles (polymersomes) in aqueous solutions was investigated using a variety of experimental techniques including optical microscopy, confocal laser scanning microscopy, dynamic/static light scattering, transmission electron microscopy, and fluorimetry. Polymersome formation was observed for all the 3-miktoarm polymers tested in a much broader range of the PEG volume fractions (0.2-0.7) than their linear diblock counterparts (0.2-0.4). Furthermore, the water-soluble anticancer drug doxorubicin hydrochloride was successfully encapsulated into the fabricated nano-sized polymersomes, and sustained in vitro release of the loaded drug was observed. Finally, possible mechanisms for the superior vesicle-forming capability of the 3-miktoarm architecture were discussed based on both the geometric and thermodynamic viewpoints.