AbstractPlasma electrolytic oxidation (PEO) technology is widely used for coating of light metals and their alloys because of the excellent coating properties it provides and its non-line-of-sight treatment which allows forming coatings on substrates with complex geometry. However, non-uniform coating thickness and surface properties may be an issue at different locations on substrates with complex shapes. In order to understand the effect of substrate geometry on PEO coating formation and uniformity, AM50 magnesium alloy specimens with drill holes of various diameter to length ratios were coated. PEO coatings were applied on these substrates in an alkaline electrolyte at constant voltage. Phase composition, coating morphology, thickness and elements distribution were studied at different locations of the drill holes. Complementary, a 3D model describing the processing is built to simulate and to predict the effect of substrate geometry on the anodic current distribution and coating formation using finite element analyses. The model can provide useful information for predicting coating growth and uniformity towards optimized PEO process design.