Calcium phosphate (Ca-P) coatings were deposited on AZ31 magnesium by hydrothermal treatment t at various pH o improve the corrosion resistance and biocompatibility of magnesium alloys. Crystal phase, morphology and composition of the coatings were investigated by XRD, SEM and EDS, respectively. The electrochemical measurements and immersion tests in Hank’s solution were performed to examine the bio-corrosion behaviors of the coated specimens. The results showed that the pH value of solutions influenced the phase composition and microstructure of the coating. And the degree of protection afforded by Ca-P coating varies with their microstructure. The deposited Ca-P coating treated at pH6 was composed of Mg3(PO4)2 and OCP. When the pH values increased, only the HA was formed. The HA coating with nano-scale and rod-like crystals prepared at pH10 can prevent the penetration of solution more effectively than the OCP at pH6 and honeycomb-like HA coating at pH8.