A variety of novel single phase solid solution as-cast Mg-RE based alloys have been developed. The as-cast alloys exhibit high ductility at room temperature. Mg-0.32Y-0.46Gd-0.34Dy-0.16Zr (wt.%) alloy exhibits high ductility expanding known performance boundaries of as-cast Mg alloys. The average tensile elongation and compression ratio at room temperature is over 30%. Meanwhile, the yield strength of tension is nearly the same as that of compression. The remarkable ductility is mainly associated with homogenous deformed microstructure, fine grain size and low mechanical anisotropy. These new developed alloys provide some implications for the development of high performance deformed Mg-based components in the future.