In this study, hydroxyapatite (HA) was coated by sol-gel method on Ti-6Al-4V alloy substrates and on micropolished surface after NaOH treatment. The former shows the cracks and poor bonding of HA whereas the latter shows coating without cracks and higher bonding strength. The higher bonding strength on NaOH treated sample could be attributed to the formation of thin sodium titanium oxide layer, the high surface roughness and high interfacial bonding between HA and substrate. © 2007 Institute of Minerals, Minerals and Mining.