Hydroxyapatite (HA) coating on AZ31 alloy substrate was prepared by a cathodic electrodeposition method. The as-deposited specimen was then post-treated with hot alkali solution to improve the corrosion resistance and bioactivity for implant applications. The microstructure and composition of HA coating, as well as its degradation behavior in simulated body fluid (SBF) were investigated. It reveals that the as-deposited coating consists of dicalcium phosphate dehydrate (DCPD, CaHPO4 center dot 2H(2)O) and HA. While 10 mu m-thick nanowhisker HA coatings doped with Na+, Mg+, HPO42 and CO32 can be found after NaOH alkali treatment, which exhibits a very similar composition of natural bone. The post-treated coating was composed of needle-like particles with 1000 nm in length and 35 nm in diameter, having a slenderness ratio of about 28.6. Electrochemical tests shows that the E-corr of Mg substrate significantly increased from - 1.6 to - 1.42 V after surface modified by HA coatings. There was obvious mass gain on post-treated specimen immersed in SBF during the first 30 days due to the Ca-P-Mg deposition. The HA-coated AZ31 alloy could slow down the degradation rate and effectively induce the deposition of Ca-P-Mg apatite in SBF, showing a good bioactivity. (C) 2008 Published by Elsevier B.V.