We investigated the hysteresis phenomenon in In-Ga-Zn-O thin-film transistors under light illumination. Hysteresis was observed under the 450-nm illumination and was increased with temperature. Light-induced hysteresis occurs due to increased subband-gap states at the interface between the gate-insulator layer and the active layer. Increased subband-gap states increase the subthreshold slope (SS) during the forward sweep. As temperature increases, the transition rate from neutral oxygen vacancy V _O to doubly ionized oxygen vacancy V _O ²⁺ increases under the same illumination conditions. The SS of the forward sweep increases with temperature due to the increase in V _O ²⁺ states.