Doxorubicin (DXR) was loaded into two kinds of thermo-responsive biodegradable star-shaped block copolymer (4arm PLGA-PEG) solutions with different PLGA block lengths under various conditions (drug-concentrations, polymer-concentrations and block compositions). All release profiles of DXR from star-block copolymer hydrogels showed sustained patterns. To assess this polymer as a sustained drug-delivery formulation, in vivo anti-tumor efficacy was examined by using tumor-bearing mice treated with DXR-loaded/DXR-free hydrogels based on the result of the release study. After I month, the mice treated with DXR-loaded S11-C1 and DXR-loaded S9-C1 copolymer solutions had remarkably suppressed tumor-volume compared to that of the mice treated with DXR-free copolymer solution. And DXR-loaded S11 hydrogel showed more significant tumor inhibition due to the different hydrophobicity between S9 and S11 copolymers. In vitro release and in vivo anti-tumor activity studies performed for 1 month revealed the potential of this hydrogel as a sustained and long-term drug delivery carrier.