Room-temperature-deposited IZTO films remained uniform amorphous phase even after annealing at temperatures of up to 350 °C, indicating that the enhancement of TFT performance was not due to the crystallization of the IZTO layer. IZTO films deposited by RF magnetron sputtering showed transparency values higher than 84%, regardless of the annealing treatment across the visible light range, which is desirable for transparent electronic device applications. It was found that annealing treatment affected TFT parameters in such a way as to increase the carrier mobility and on/off current ratio, and to decrease the sub-threshold swing value. The threshold voltage value also shifted to the negative direction, and the carrier concentration value increased upon annealing. Interface defect concentration also reduced, resulting in the movement of more electrons without being trapped between the active layer and the source or drain electrode. However, stability improvement under bias stress still remains as an issue to enhance the performance of a-IZTO thin film transistors in the near future.