The corrosion degradation behavior of a Mg-Ca alloy with high Ca content aiming for a potential bone repair material in the simulated body fluid (SBF) was investigated. The microstructure and phase constitution of the pristine Mg-30% Ca (mass fraction) alloy were characterized with scanning electron microscopy (SEM) and X-ray diffraction (XRD). The Mg-30% Ca alloy samples were immersed in the SBF for 90 d, and the morphology, composition and cytotoxicity of the final corrosion product were examined. It is found that Mg-30% Ca alloy is composed of alpha-Mg and Mg2Ca phases. During the corrosion process in the SBF, the Mg2Ca phase acts as an anode and the alpha-Mg phase acts as a cathode. The final corrosion product of the Mg-30% Ca alloy in SBF includes a small amount of black precipitates and white suspended particles. The white suspended particles are Mg(OH)(2) and the black particles are believed to have a core-shell structure. The cytotoxicity experiments indicate that these black precipitates do not induce toxicity to cells.