The photoelectrolysis of water has been investigated in a photoelectrochemical cell with the single-crystal photoanodes Nb: Sr Ti O3 (0mol%, 0.07mol%, and 0.69mol%) and the electrolyte Na2SO4s0.1M, pH=5.92d. The Nb: Sr Ti O3 anodes (0.07 mol% and 0.69 mol%) show much higher photocurrent density than the pure Sr Ti O3 anode in several orders of magnitude. The incident photon to current conversion efficiencies at 298.2 nm and an applied potential 1.5V (versus the saturated calomel electrode) for three Nb: Sr Ti O3 (0mol%, 0.07mol%, and 0.69mol%) anodes are 0.92%, 15.67%, and 4.32%, respectively. The transmittance spectra of the Nb: Sr Ti O3 show an obvious optical absorption around 516 nm (～2.4eVd). It is interpreted that two-photon process takes part in the charge transfer from the valence band to the conduction band through the intermediate states induced by Nb doping.