In this work, we examined the stability of amorphous IGZO TFT under electrical bias and light illumination. For the investigation of electrical bias effect on device stability, +20 V gate bias or -20 V gate bias was continuously applied on devices for 10000 s. For the investigation of illumination effect, we used monochromatic light source illuminating devices under the stress test. There was a specific stress condition to result in the degradation of device performance. Our IGZO TFT was considerably degraded only when the negative gate bias was applied with the light illumination. The subthreshold region was stretched in the negative direction and a humped shaped was observed in the middle of the subthreshold region. We propose two kinds of possible degradation mechanisms on the basis of experimental results and numerical simulations. The one is hole trapping in the gate insulator and the other is the change of transition level of deep donor states. The peculiar humped shape in the subthreshold region could be explained by the latter mechanism. (C) 2011 Elsevier B.V. All rights reserved.