We examined the grafting effect of Cu2+ on the photocatalytic activity of Ti-substituted hydroxyapatite (Ti:HAp). When 0.05 wt% Cu2+ was grafted on the 10%-Ti:HAp, the photocatalytic activity to acetaldehyde decomposition under UV light irradiation was increased by three times. ESR measurements revealed that valence band electrons transfer to the Cu2+ via hybridized band, which promotes electron-hole separation. However, visible light activity of Ti :HAp did not appear even after Cu2+ grafting although TiO2 was reported to become active by the grafting. This experimental result was explained by the difference in the electronic states of valence bands of Ti:HAp and TiO2. (c) 2013 Elsevier B.V. All rights reserved.