Pure Mg foams stabilized by ex-situ added CaO particles were developed in this study. Mg/xCaO foams (x = 5, 7 and 10 wt.%) exhibited uniform pore distribution, thinner yet stable pore wall cross-sections. Mg-Ca-O transition phase and MgO particles were formed at the interface of Mg-CaO, which improved the wetting of CaO particles in the Mg melt. The CaO particles, Mg-Ca-O transition phase and blocky MgO particles collectively stabilized the foam. Mg-Ca-O and MgO phases disperse along the gas-liquid interface of foams thereby preventing from wrinkling of interfaces during solidification. TEM analysis of Mg/10wt.% CaO foam powder also confirmed the formation of nano-sized (~ 200 nm) MgO particles of different morphologies. TG-DSC analysis confirmed the exothermic Mg-CaO reaction at 610 ºC, resulting in formation of Mg2Ca and MgO phases, as identified using XRD analysis. 7 wt.% CaO addition exhibited the best foam structure in terms of mean pore diameter (2.19 mm) and circularity (0.75). The lowest foam density of 0.38 g/cm3 and relative density of 21 % was achieved in case of Mg/10wt.% CaO foams.