Magnesia-Calcia materials containing 22-53 wt% CaO have been investigated for refinery slag corrosion and penetration resistance by Scanning Electron Microscopy (SEM) and XRD. The corrosion- and penetration -resistance of MgO-CaO materials is better and is enhanced with the increase of CaO content in specimens. It can be explained that, with increase of CaO content, the specimens reacted with slag form higher melting point minerals of C3S and C2S, creating a dense surface layer, which stopped others from penetrating further. Thus, the corrosion and penetration resistance was improved. No matter what kinds of slag, the depth of penetration decreased with increase of CaO content.