Effect of CaO addition on age hardening response has been studied by using optical microscopy, scanning electron microscopy and differential thermal analysis in AZ91 and CaO-containing ECO-AZ91 alloys. After solution treatment, the ${eta}$($Mg_{17}Al_{12}$) phase formed during solidification mostly disappeared in the microstructure in the AZ91 alloy, whereas numerous ${eta}$ precipitates containing Ca were still observed in the ECO-AZ91 alloy due to its enhanced thermal stability. The ECO-AZ91 alloy showed the delayed peak aging time and higher peak hardness compared with those of the AZ91 alloy. The activation energies for ${eta}$ precipitation calculated by means of Kissinger method increased from 71.4 to 85.6 kJ/mole by the addition of CaO, which implies that CaO plays a role in reducing ${eta}$ precipitation rate in the AZ91 alloy.