Abstract: Corrosion properties and mechanism of ZM5 magnesium alloy and ZM5-0.1RE(RE was mischmetal) magnesium alloy in 3.5wt% NaCl solution were investigated by immersion test, phase analysis and microstructure observation. The results indicate that after adding 0.1wt%RE in ZM5 magnesium alloy, the micostructure was refined, β-Mg17Al12 phases distributed discontinously and dispersively and the corrosion rate reduced. The generation of the rare earth compounds Al3Ce phase and Al11La3 phase reduced the segregation level of Al in α-Mg grains which was the main reason of the corrosion resistance increase.