A series of four-arm star block copolymers poly(ethylene glycol)-block-poly(d,l-lactide) (PEG-PDLLA) with different hydrophobic length were synthesized by ring-opening polymerization of lactide and characterized using 1H NMR and gel permeation chromatography (GPC). These copolymers could self-assemble in aqueous solution to form the vesicle structure with controlled size and morphologies. Transmission electron microscopy (TEM) and DLS show the polymersomes are spherical with diameter of 70∼500 nm. The polymersomes made by direct hydration of copolymer thin films in water exhibit the controllable ability of encapsulating hemoglobin under mild condition. The hemoglobin content in the polymersomes could reach to 35 wt %. More importantly, the encapsulated hemoglobin keeps its own bioactivity and is capable of binding oxygen. This hemoglobin-encapsulated four-arm PEG-PLA polymersomes could have the potential to be applied as an artificial oxygen carrier for transfusion. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40433.