KOH addition was introduced into the electrolyte and ceramic coating has been fabricated on magnesium alloy. Effect of KOH on MAO process and corrosion resistance was focused on. The results show that the introduction of KOH could lower the arcing voltage and working voltage of MAO process. However a too high concentration of KOH will lead a high arcing voltage. With KOH additive in the electrolyte the number of micropore decreases and the surface porosity increases. In order to get a higher coating forming velocity and better corrosion resistance, the concentration of KOH is 1 ~ 3 g / L.