Tensile properties of AZ91D magnesium alloy coated by Plasma Electrolytic Oxidation method for various coating times were investigated. The coatings on AZ91D magnesium alloy showed porous microstructure, and became thicker with increasing the coating time. After tensile test, the coated AZ91D magnesium alloy had lower tensile strength and elongation than the uncoated one. The tensile strength and elongation decreased with increasing the coating time, while the proof stress was almost no changed. The uncoated AZ91D magnesium alloy revealed ductile fracture surface. As the coating time increased, however, the fracture surface was changed to brittle fracture morphologies.