In this study, plasma treatment using Ar and H2 and plasma polymerization with trimethylsilane (TMS), were carried out to create the interface film systems with a structure of Mg/plasma interlayer/cathodic electrophoretic coating (E-coat) for a magnesium (Mg) alloy AZ31B. The interface adhesion of the coating system was evaluated using N-methylpyrrolidinone (NMP) paint removal test and ASTM tape test conducted under dry and wet conditions. Plasma TMS film could stick the Mg substrate more strongly than E-coat.The surface of samples with different interlayer treatment after NMP test has been observed by SEM. The potentiodynamic study performed on the E-coated samples showed a noticeable reduction of corrosion current, three orders of magnitude less than bare Mg. Salt spray test also was done to confirm the resistance corrosion of E-coat. It was found that plasma interface treatment has the ability of increasing the adhesion of E-coat on Mg alloy more or less, thus restrict access to damaging species that exist in Cl- aqueous environments.