The as-extruded X20 magnesium alloys containing 2.0wt% Ca were solution-treated at 410 oC for 20 h. The microstructures before and after solution treatment were observed by optical microscope (OM), scanning electron microscope (SEM) together with an energy dispersive spectroscope (EDS), and transmission electron microscope (TEM). The phase compositions before and after solution treatment were confirmed by X-ray diffraction (XRD). The results show that the amount of β-Mg17Al12 phases decreases remarkably after solution treatment, and new Al2Ca phases are formed. The formation of Al2Ca phase might be due to high concentration of Ca atoms in residual β-Mg17Al12 phases, which leads the place of Mg atoms to be substituted by Ca. On the other hand, Al2Ca phase could be also described as a completing product between Al-Ca system and Mg-Ca system intermetallics due to its high structural stability