The Ni in the Ni/n-Si photoanode can not only protect Si from corrosion, but also catalyze the water oxidation reaction. However, the high density of interface states at the Ni/n-Si interface could pin the Fermi level of silicon, which will lower the Schottky barrier height of the Ni/n-Si. As a result, a low photovoltage and consequent high onset potential of Ni/n-Si photoanode for water oxidation were generated. In this study, the interfacial states of the Ni/n-Si photoanodes were efficiently diminished through a rapid thermal process (RTP). Calculated from the Mott–Schottky plots, the Schottky barrier height of Ni/n-Si was increased from 0.58 to 0.78 eV after RTP. Under the illumination of 100 mW cm–2 of the Xe lamp, the onset potential of the Ni/n-Si photoanode for water oxidation was negatively shifted for 150 mV after RTP. Besides, the RTP-treated Ni/n-Si photoanode exhibited a high stability during the PEC water oxidation of 8 h in 1 M KOH solution.