The paper demonstrated the anti-corrosion properties of an AZ91 Mg alloy fabricated via microarc discharge oxidation under an AC condition with various pulse frequencies ranging from 60 Hz to 2000 Hz. The structural features and phase compositions of the coating layers were characterized by utilizing scanning electron microscope and X-ray photoelectron spectroscopy. The optimum frequency for the coating layers with thick, uniform thicknesses was found to be 500 Hz for the conditions used in this study. This fact was explained by the change in the mode of spark discharge during coating, resulting in the occurrence of surface cracks as well as the transformation of Mg(OH)₂ to MgO. Potentio dynamic polarization tests in 3.5 mass% NaCl solution revealed that the sample coated at a frequency of 500 Hz exhibited a corrosion potential of −1.42 V (vs. Ag/AgCl), suggesting a better corrosion resistance as compared to the other conditions.