Without any surfactants or special equipments, a novel series of efficient photocatalysts (Na, K)TaO₃ (NKT) is successfully synthesized via a simple and convenient molten-salt process. The synthesized NKT is determined to be highly crystallized single crystal nanocubes with a diameter of 100 nm. A detailed time-resolved TEM kinetic study of the formation of the structure is carried out to investigate the growth mechanism of nanocubes. Photocatalytic activity of NKT is found to be especially high without any co-catalysts. However, stability of NKT is unsatisfied due to the forming of peroxidation phases. Doping tetravalent Zr⁴⁺ and Hf⁴⁺ (NKZT and NKHT) efficiently improves both activity and stability of catalyst without changing energy level. Uniform and nearly-monodispersed nanocubes with a size of about 50 nm are observed on NKZT and NKHT. Without co-catalyst, photocatalytic activity achieves 4.65 and 2.31 mmol h⁻¹ for H₂ and O₂ generation, respectively. The reason of especially high activity of this series is believed to be the prolongation of lifetime for photo-excited charges.