Although Mg−Al−RE (RE: rare earth)alloys have significant weight saving potential in automotive industries, their applications were interrupted due to their unsatisfactory mechanical performance. Intermetallic phases in structural magnesium (Mg)alloys are of practical significance for being able to optimize their microstructures for specific applications. Here we report a new alloy design concept that can develop an alloy for a given system with out-standing mechanical performance and low cost. Such designed alloy exhibits even more excellent strength-ductility balance and cost creep-performance than the commercial/experimental die casting Mg alloys and A380 aluminum alloy. The alloy design concept used in this work is based on strictly controlling intermetallic phase components according to modifying alloy's compositions. Such developed alloy has huge potential to be widely applied in automotive powertrain components acted as the alternative of Mg alloys and even aluminum alloys.