Abstract: The corrosion resistance of as-extruded AZ31 magnesium alloy plate was evaluated by electrochemical test, and the corrosion behavior of ED-ND and TD-ND section of as-extruded AZ31 magnesium alloy plate (ND, TD, ED is thickness, width and extrusiondirection respectively) in 3.5% NaCl solution was compared.The results show that the grain size of the TD-ND section of the as-extruded AZ31 magnesium alloy plate is smaller.The corrosion of as-extruded AZ31 magnesium alloy plate on the two sections is gradually changed from local filamentary corrosion to point corrosion.Compared with those of ED-ND section, the corrosion product on TD-ND section is much uniform and dense.The self-corrosion potential value of TD-ND section is-1.31 V, which is higher than that of the ED-ND section.The TD-ND section has higher corrosion resistance.Because the grain refinement leads to the uniformity and compactness of passivation film on the TD-ND section of the alloy, therefore reduces the electrochemical corrosion on the section of the magnesium alloy.