The surface potential of the TiO2 scale formed on Ti was controlled by varying the Ti heat treatment conditions in a N2 atmosphere containing a trace amount of O2. The surface potential was attributed to the effective charge of nitrogen-related defects in the TiO2, where the positive and negative surface charges were associated with (N2)0 +2 and (N)0 -1, respectively. The latter defects were formed only during the early stages of the heat treatment, and with increasing treatment time, this was followed by the formation of voids containing N2 in the scale rather than the disappearance of the defects in the TiO2 crystal lattice, resulting in zero surface charge. Hydroxyapatite (HAp) formation and osteoconductivity were enhanced on nitrogen-doped TiO2 scale with either a positive or negative surface potential. In contrast, for the unchanged TiO2 scale, no HAp formation was observed and the osteoconductivity was low.