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Microstructure, bio-corrosion properties of as-cast Mg-2Zn-0.5Ca-Y series alloy was investigated for biomedical application in detail by optical microscopy, scanning electronic microscopy(SEM) , immersion tests and electrochemical measurements in NaCl solution (3.5wt.％) and Hank''s solution. The morphology of the Mg-2Zn-0.5Ca-Y series magnesium alloy shows that the corrosion behavior is not uniform and corrosion layer with many cracks were on their surface after immersion tests . The corrosion resistance of as-cast Mg-2Zn-0.5Ca-Y series alloy increased with the addition of the rare earth element Y and Mg-2Zn-0.5Ca-1.0Y alloy is the best. All of the potentiodynamic polarization curves obtained in the tests in Hank''s solution and NaCl solution showed current plateaus which were due to the presence of a protective corrosion product film. The electrochemical impedance curves of the alloy in the test solutions had only one capacitive loop. The appearance of a capacitive loop at high frequency was due to the existence of stable double-layer capacitance. The capacitive loop at low frequency was attributed to diffusion control at film-free areas.