Powders of NaTaO3 doped with alkaline earth metal ions were prepared. Scanning electron microscopy observation revealed that NaTaO3 crystals doped with Ca, Sr, and Ba became smaller than nondoped NaTaO3 crystals. In addition, nano-step structures were constructed on the surfaces of NaTaO3 doped with Ca, Sr, and Ba. The photocatalytic activity of water splitting on NaTaO3 was improved by the doping of Ca, Sr, and Ba. It is due to the small particle size and the formation of surface nano-step structures as observed for La doping.