Effects of illumination on the electrochemical behavior of selenium electrodeposition on indium-doped tin oxide(ITO) glass are investigated by linear sweep photovoltammetry, chronoamperometry, electrochemical impedance spectroscopy, and potentiostatic electrodeposition. Linear sweep photovoltammetry results show that the illumination can both enhance the four-electron reduction of Se(IV)/Se(0) and the six-electron reduction of Se(IV)/Se(-II). Se(IV)/Se(-II) and Se(IV)/Se(0) redox potentials are all positive than the conduction band of selenium, thus the photo-generated electrons which induced from the valence band of selenium can flow to the electrolyte to accelerate the reduction of Se(IV)/Se(-II) and Se(IV)/Se(0). Chronoamperometry measurements demonstrate that the electrocrystallization of selenium follows a 3D progressive nucleation model in the dark, while it adheres to a 3D instantaneous nucleation model when under illumination. This is due to the additional driving force (generated by photo-induced electrons) for reduction of SeO32-. Electrochemical impedance spectroscopy proves that the Faraday resistance of Se(IV)/Se(0) reduction can be greatly reduced by the light. Potentiostatic electrodeposition results indicate that the illumination can accelerate the deposition rate. A denser and larger particle size of selenium thin films can be obtained after applying the photo-assistance. (C) 2017 The Electrochemical Society. All rights reserved.