To explore the effects and differences of Ce and Y rare earth elements on corrosion performance of Mg-Mn-RE magnesium alloys, the Mg-Mn-Ce and Mg-Mn-Y alloys with Ce and Y contents of 0.5, 1.0, 1.5 and 2.0 wt.% were prepared and the corrosion performance and corrosion type of the alloys were investigated by immersion corrosion test in 3.5% NaCl solution. The mechanisms of corrosion and corrosion inhibition of the alloys were analyzed by measuring their electrochemical polarization curve, AC impedance spectrum and micro-area potential distribution. Further, the effects of Ce and Y elements on the corrosion of alloys and their differences were discussed based on the change of microstructure of the alloys. The results indicated that the corrosion rate of Mg-Mn-Ce and Mg-Mn-Y alloys decreased with the increase of immersion time. When the contents of Ce and Y were 1.0 wt.% and 0.5 wt.% respectively, the corrosion weight loss after immersion for 1 day and average daily corrosion weight loss for 7 days of the alloys were the smallest. Almost all Ce in Mg-Mn-Ce alloy formed Mg12Ce compound and distributed in grain boundary in the form of eutectics. The potential of the compound was lower than that of α-Mg phase, which could form the galvanic cell with the α-Mg phase. As a result, micro-galvanic corrosion occurred preferentially in grain boundary and Mg-Mn-Ce alloy showed a typical intergranular corrosion type. With the increase of Ce content in Mg-Mn-Ce alloy, the amount of compound increased, and the degree of intergranular corrosion was aggravated accordingly. Mg-Mn-Y alloy was mainly dominated by the anodic dissolution of matrix. However, Y in Mg-Mn-Y alloy mainly dissolved in α-Mg phase and caused stress in α-Mg solid solution and the corrosion of matrix began preferentially at the active points of corrosion with higher stress. Mg-Mn-Y alloy showed a type of uniform corrosion with micro pits. With the increase of Y content in the alloy, the density of active point caused by stress increased, and the corrosion rate of the matrix became faster. In addition, Y exceeding the dissolution limit also formed Mg24Y5 compound and caused the micro-galvanic corrosion in Mg-Mn-Y alloy. Both Ce and Y elements could improve the film resistance Rfilm and the charge transfer resistance Rt of the alloys. However, Ce played a more significant role in increasing the membrane resistance by increasing the compactness of corrosion product film while Y in increasing the charge transfer resistance by increasing the potential of matrix respectively, so as to slow down the corrosion of the alloys. The maximum film resistance Rfilm of Mg-Mn-Ce alloy and the maximum charge transfer resistance Rt of Mg-Mn-Y alloy appeared when the content of Ce was 1.0 wt.% and Y was 0.5 wt.% respectively. The content of Ce affected the degree of micro-galvanic corrosion and product film protection of Mg-Mn-Ce alloy, while the content of Y affected the matrix potential, the anodic dissolution of matrix and the degree of micro-galvanic corrosion of Mg-Mn-Y alloy, resulting in that the corrosion performance of the alloys was affected by the content of rare earths and the optimal content of rare earths. Thus, the main corrosion and corrosion inhibition mechanisms of Mg-Mn-Ce and Mg-Mn-Y alloys are obviously different due to difference of the microstructure of the two alloys and the action of Ce and Y elements in corrosion of the alloys.