Electrolytic plasma oxidation (EPO) has been considered as a new technique to form ceramic coatings on magnesium alloys for corrosion protection. In this study, a magnesium alloy, AM50, was treated by EPO, and the Taguchi experimental analysis method was used to systematically investigate the effects of four factors (treatment time, current density, and concentration of KOH and NaAlO2) with three levels on the thickness and corrosion resistance of the coatings. It was found that treatment time was a major factor affecting the thickness, while the KOH concentration had the most significant effect on the corrosion resistance of the coatings. The phase compositions, thickness and uniformity of the coatings were responsible for the differences of the corrosion resistance of the coatings. With an increase of the KOH concentration, the MgO phase was enriched, and the MgAl2O4 phase was reduced in the coatings. On the other hand, increasing the NaAlO2 concentration resulted in an enrichment of the MgAl2O4 in the coatings. It was also found that the coatings grew towards both their substrates and their external surface, and unevenly distributed sparks lead to non-uniformity in coating thickness. (c) 2005 Elsevier B.V. All rights reserved.