The activities of water splitting over NiO/NaTaO3 photocatalysts were improved by doping of lanthanides, La, Pr, Nd, Sm, Gd, Tb and Dy into NaTaO3. Lanthanum was the most effective as a dopant. The apparent quantum yield at 270 nm amounted to approximate to 50%. Moreover, the deactivation observed for a non-doped NiO/NaTaO3 photocatalyst was suppressed. SEM observation suggests that these effects on the photocatalytic properties are mainly due to the decrease in the particle size and the characteristic step structure created by the doping.