The microstructure and corrosion resistant property of AZ91 magnesium alloy with different additions of Sc were investigated The microstructures were observed by OM and SEM, and the corrosion properties were studied by electrochemical technique. The results show that with the increase of Sc content in the range from 0.18%-0.57%, the alloy microstructures are fined continuously, and the corrosion resistance of the alloy is improved. Although the change of the corrosion voltage is not big, the corrosion current density increases obviously. When Sc content is 0.39%, the minimum current density appears.