We have investigated a stability of oxide TFTs (IGZO) under electrical bias and monochromatic light illumination. Under -20 V gate bias, when 650 nm monochromatic illumination with intensity of 1 mW/cm2 was radiated, threshold voltage (Vth) was shifted by about -1.55 V while V th was almost constant without illumination. Even though the photon energy of 650 nm is much smaller than optical band gap of oxide active layer (Eopt∼3.0 eV), Vth was changed due to generation of carriers through localized state. However, under positive gate bias of 20 V, Vth was not changed with and without 650 nm light illumination. In terms of 300 nm wavelength light, Vth was shifted positively under negative bias stress and negatively under positive bias stress. It is considered that light affected not only the active layer but also the gate insulator layer.