There have been many alloy systems for the Mg based bulk metallic glasses (BMGs), with the previous optimum composition being Mg65Cu 25Y10. In this study, new optimum alloy designs are made based on the theoretical model involving the electron per atom e/a-related criterion and the recent model of optimum composition extension from the binary eutectic-pair criterion. Both models suggest that the optimum Mg based BMGs might possess a composition with a lower amount of Mg element. It follows that a series of Mg based BMGs with 50-65 at% Mg and 10-25 at% rare earth element are prepared. The glass forming ability, thermal characteristics, and mechanical performance are examined and discussed.