CaO addition to Mg forms the Al2Ca and Mg2Ca phases without solid solution even below solubility limit. In this study, thermodynamic considerations of CaO added Mg alloys in recycling process are investigated in comparison with Ca added Mg alloys. Thermodynamic considerations were done by using the army-navy-air force (JANAF) thermochemical tables between Ca or CaO added Mg alloys and fluxes such as MgCl2, KCl, NaCl and MnCl2. The mechanisms during recycling process were thermodynamically discussed. Actual recycling experiments were also carried out to compare the thermodynamic results.