AZ91D magnesium alloys were processed in the base electrolyte composed with sodium hydroxide and sodium silicate, not containing or containing potassium fluoride respectively, by micro arc oxidation (MAO). Influence of potassium fluoride on growth, microstructure and corrosion resistant of the coatings was investigated. The results show that after potassium fluoride was added to the base electrolyte, the breakdown voltage to the specimen decreased but the breakdown process was more vigorous to lead to formation of larger sparks on the specimen surface during micro arc oxidation. As a result, the grow rate of the coating significantly increased and the obtained coating had a remarkable improvement in terms of coating thickness. Meanwhile, the surface porosity of the coating somewhat increased but the amount of the micro-pores on the coating surface decreased. The addition of potassium fluoride into the electrolyte is helpful for the formation of MgF₂and MgAl₂O₄ phases of the coating. Compared with silicate and oxygen elements from the electrolyte, F_- ion is more likely to be adsorbed by magnesium in the substrate and also easily overcomes the obstacle of the generated coating to migrate into the inner of the coating. The corrosion resistance of the coating treated in the electrolyte with potassium fluoride is increased under the influence of significant increasing in coating thickness mainly and the existence of favorable phases such as MgF₂ newly born and more MgAl₂O₄.,