Photocatalytic activity of KTaO3 catalyst to water splitting is much improved by organic dye-modification. In particular, cyanocobalamin (Vitamin B12) is the most effective for improving water splitting activity, and the formation rate of H2 and O2 are achieved values of 1912 and 979 μmol gcat-1h-1, respectively, under the light irradiation from Xe lamp (300W). Mechanism of photocatalytic H2O splitting into H2 and O2 on PtOX/KTa(Zr)O3 modified with various dyes were investigated. Photoluminescence (PL) spectra of KTaO3 catalysts suggested that the excitation energy transfers from KTaO3 to Cr- Tetraphenylporphyrin (Cr-TPP) and PtOX co-catalyst. Transient response of photovoltaic potential revealed that the excitation state of KTaO3 was enlarged over 500 jis by dye-modification. Photo-excited electron in KTaO3 transfers to dye, and a long charge separation state was achieved. Thus, it was suggested that the photocatalytic activity was much improved by increasing lifetime of photo-excited charges which can reach to the reaction site effectively.