The enhancement of the electrical conductivity by doping is important in hematite (α-Fe(2)O(3)) photoanodes for efficient solar water oxidation. However, in spite of many successful demonstrations using extrinsic dopants, such as Sn, Ti, and Si, the achieved photocurrent is still lower than the practical requirement. There is still lack of our understanding of how intrinsic oxygen defects can change the photocurrent and interact with the extrinsic dopants. In this study, we systematically investigate the interplay of oxygen vacancies and extrinsic Sn dopants in the context of photoanodic properties. As a result, we demonstrate that the controlled generation of oxygen vacancies can activate the photoactivity of pure hematite remarkably and further enhance the Sn doping effects synergistically. Furthermore, the correlated behavior of oxygen vacancies and Sn dopants is closely linked to the variation of electrical conductance and results in the optimum concentration region to show the high photocurrent and low onset voltage.