The effects of increase extent of voltage on microstructures and corrosion resistance of micro-arc oxidation (MAO) coatings for AZ91D magnesium alloy in silicate electrolyte were investigated. The results showed that with increase of the voltage increase extent, all of the thickness, porosity and bonding force of MAO coating first increased, and then decreased. These parameters were all up to their maximum values when the voltage increase extent was 20 V. However, the roughness of the coatings was always increased with the increasing of voltage increase extent. The coating had the optimum corrosion resistance when voltage increase extent was not below 25V.