Antibacterial behaviors of magnesium (Mg) based metal, pure Mg and AZ31 alloy, with and without surface coatings were studied. The results indicated that both pure Mg and AZ31 alloy had strong antibacterial effect against Escherichia coli and Staphylococcus aureus with rapid increases of pH values of the bacterial suspensions. Pure Mg with porous silicon-contained (Si) coating by micro-arc oxidation still maintained its antibacterial ability while with a mild increase of the pH value. However, pure Mg and AZ31 alloy with fluorine-contained (F) and Si coatings by chemical conversion, respectively, lost their antibacterial abilities with nearly no change of the pH values, owing to the much dense coatings on surfaces.