Magnesium alloys are newly promising biomaterials with potential application of human bone replacement. However, there is a drawback due to their high corrosion rates. In this study, AZ31 magnesium alloys were coated using microarc oxidation (MAO) process. Two oxidation durations, 1 minute and 5 minutes, were used. The samples were immersed in the simulated body fluid (SBF) for up to seven days. Then the electrochemical behaviors of the two samples were comparatively investigated. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) experiments were used. The results show that the 5-minute MAO coated sample had a better corrosion resistance than the 1-minute MAO coated sample. The study shows processing parameters, e.g., oxidation time, can be used to design an optimized MAO-coated magnesium alloy with controlled corrosion rates.