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Nano-sized biocompatible and biodegradable polymersomes were prepared based on poly(D,L-lactide)block-poly(2-methacryloyloxyethyl phosphorylcholine) (PLA-b-PMPC) diblock copolymers and applied for the release anti-cancer drugs. Hydrophobic doxorubicin (DOX) and hydrophilic doxorubicin hydrochloride (DOX$HCl) were successfully loaded into the polymersome membrane and polymersome interior, respectively. The in vitro release studies demonstrated that the release of DOX and DOX$HCl from polymersomes was highly pH-dependent, i.e. significantly faster drug release at mildly acidic pH of 5.0 compared to physiological pH 7.4. Furthermore, DOX$HCl-loaded polymersomes exhibited faster drug release than DOX-loaded polymersomes under the same pH conditions. The highly pH-depended release behavior was attributed to the hydrolysis of PLA-b-PMPC, which would result in morphological transformation from polymersome to micelle with a triggered release of the encapsulated drugs. The drug-loaded polymersomes were shown to rapidly enter HepG2 cells, localize in their endosome/lysosomes with acidic pH environment and display enhanced intracellular release of the drugs into the cytosol. These biocompatible and acid pH-sensitive polymersomes might have great potential for cancer therapy.