K(2)LnTa(5)O(15) (Ln=La, Pr, Nd, Sm, Gd, Tb, Dy, and Tm) loaded with NiO co-catalysts showed photocatalytic activities for water splitting into H-2 and O-2 in stoichiometric amounts under UV light irradiation (hv > 3.9-4.1 eV). The photocatalytic activities strongly depended on the lanthanide ions. Among them, K2PrTa5O15 and K2SmTa5O15 showed relatively high activities.