NiO-loaded NaTaO3 doped with lanthanum showed a high photocatalytic activity for water splitting into H-2 and O-2 in a stoichiometric amount under UV irradiation. The photocatalytic activity of NiO-loaded NaTaO3 doped with lanthanum was 9 times higher than that of nondoped NiO-loaded NaTaO3. The maximum apparent quantum yield of the NiO/NaTaO3:La photocatalyst was 56% at 270 nm. The factors affecting the highly efficient photocatalytic water splitting were examined by using various characterization techniques. Electron microscope observations revealed that the particle sizes of NaTaO3:La crystals (0.1-0.7 mum) were smaller than that of the nondoped NaTaO3 crystal (2-3 mum) and that the ordered surface nanostructure with many characteristic steps was created by the lanthanum doping. The small particle size with a high crystallinity was advantageous to an increase in the probability of the reaction of photogenerated electrons and holes with water molecules toward the recombination. Transmission electron microscope observations and extended X-ray absorption fine structure analyses indicated that NO cocatalysts were loaded on the edge of the nanostep structure of NaTaO3:La photocatalysts as ultrafine particles. The H-2 evolution proceeded on the ultrafine NiO particles loaded on the edge while the O-2 evolution occurred at the groove of the nanostep structure. Thus, the reaction sites for H-2 evolution were separated from those of O-2 evolution over the ordered nanostep structure. The small particle size and the ordered surface nanostep structure of the NiO/NaTaO3:La photocatalyst powder contributed to the highly efficient water splitting into H-2 and O-2.