The microstructure and mechanical properties of as‐cast Mg96Er3Cu1, Mg96Er3Ni1, and Mg96Er3Zn1 alloys prepared by conventional ingot metallurgy are investigated. Results show that the bulk 18R long period stacking ordered (LPSO) phases in all three as‐cast alloys solidify directly from the melt. The grain size of the Mg96Er3Zn1 alloy is the smallest; the volume fraction of the LPSO phase in the Mg96Er3Ni1 alloy is the highest in all three alloys due to negligible Ni solid solubility in α‐Mg. The thermal stability of the 18R LPSO phase in the as‐cast Mg96Er3Ni1 alloy is much higher than in the other two alloys. Tensile tests indicate that the as‐cast Mg96Er3Zn1 alloy shows the best ductility; the as‐cast Mg96Er3Ni1 alloy possesses the highest tensile strength, and the ultimate tensile strength and yield tensile strength are 245 and 149 MPa, respectively, accompanied by a good elongation of 7.1%.