Microstructure and corrosion behavior of 0.2wt% Ca and/or 0.2wt% Y modified Mg-2Zn-1Al (ZA21) rolled sheets were analyzed by SEM, XRD, hydrogen evolution and electrochemical measurement. Results show that Ca and Y refine the grains, modify the second phases, and reduce the Mn content in Mn-containing phase. In 3.5wt% NaCl solution, the preferential corrosion sites locate at the Mg-matrix near Mn-containing phases. The 12 h-corrosion rates satisfy ZA21 (8.59 mm/a)>ZA21+0.2Ca (7.17 mm/a)>ZA21+0.2Y (4.22 mm/a)>ZA21+0.2Ca+0.2Y (1.26 mm/a). The enhanced corrosion resistance of the Ca, Y modified alloys can be accredited to the follows: (1) the refinement of grain size; (2) the devitalization of the intensity of the micro-galvanic corrosion by generating low-Mn and non-Mn phases rather than the high Mn-content phases; (3) the dense, shallower cracked and more protective corrosion product film composed of Mg, Mg(OH)₂, Al₂O₃, Ca-containing and Y-containing components replaces the fully-cracked and limited protective one without Ca, Y incorperation.