ABSTRACTTiO₂ nanostructures with various morphologies and crystal structures were obtained by calcination of alkaline hydrothermal synthesized hydrogen titanate at different temperatures. The photocatalytic activities of the as-prepared samples were investigated by degradation of methyl orange aqueous solution under ultraviolet irradiation. The effects of the phase composition, crystallinity, surface area and shape of the nanostructures were evaluated. The results showed that high crystallinity, pure anatase phase and nanowire structure are favorable for the photocatalysis. The dependence of the photocatalytic activity on the surface area is not significant as usually expected.