Octa-calcium-phosphate (OCP) coatings were formed at weak acid on pure magnesium by a hydrothermal treatment. The microstructure of the coatings was investigated with XRD, SEM and TEM. At initial stage of treatment, a dual-layer coating consisting of an inner dense layer and an outer layer composed of plate-like crystals was formed. Both the layers grew with an increase in the treatment time up to 7.2 ks. Further increase in the treatment time caused the following change in the coating; (1) suppression of the coating growth, (2) partial change of OCP in the inner layer into hydroxyapatite (HAp) and (3) growth of a Mg(OH)2 layer at the interface between the coating and the Mg substrate. The adhesive strength of the coatings decreased with the growth of the Mg(OII) layer. The corrosion resistance of the specimens in a 3.5wt% NaCl solution increased with the growth of the coating.