The influence of the amount and type of long-period stacking ordered (LPSO) phase on the corrosionCorrosion behavior of both the as-cast and heat-treated Mg–15Dy–1.5Zn alloys in 0.9% NaCl solution was investigated. It was found that the network structure 18R-LPSO phase is an effective barrier to further corrosion of the as-cast sample. After T4 treatment for 24 h, the dendrites disappeared and part of 18R-LPSO dissolved in the matrix, which weakened the corrosion protection. Meanwhile, such LPSO phase acts as a cathodic to accelerate the corrosionCorrosion of the matrix because of its potential difference from the magnesium matrix. After T4 treatment for a longer time, 18R-LPSO phase could transform into 14H-LPSO phase which has a different effect on corrosion. The galvanic corrosion also occurs between the 14H-LPSO phase and the matrix. Its uniform and dense distribution results in the formation of continuous corrosionCorrosion products on the surface, which is beneficial for corrosion resistance.