Hydroxyapatite (HAp) coatings were uniformly formed on pure Mg by a hydrothermal treatment using a C10H12N2O8Na2Ca (Ca-EDTA) solution. The growth mechanism of the HAp coating was investigated with XRD, SEM and TEM. At the initial stage, dome-shape HAp precipitates were formed on the Mg. Subsequently, the precipitates grew and the coating became a dual-layer consisting of an inner dense HAp layer and outer course layer consisting of rod-like HAp crystals. The protectiveness of the coatings with different treatment times was investigated by a polarization test in a 3.5 wt.% NaCl solution. The corrosion current density decreased with the growth of the HAp coating. (C) 2011 Elsevier B. V. All rights reserved.