We investigated the stability of amorphous InGaZnO (a-IGZO) thin film transistor (TFT) under pulsed gate bias stress with light illumination, and compared that with DC bias stress. In the case of DC gate bias stress, a-IGZO TFT showed considerable degradation only when the negative gate voltage was applied under light illumination. For the pulsed gate bias stress, we constructed a voltage square wave with a bi-level of -20 V and 0 V and also made variations in the time duration of -20 V level in a cycle. Although the accumulated time duration of applying -20 V in the pulsed gate bias stress was the same with that of DC -20 V stress, the degradation was much reduced compared with DC bias stress. On the basis of the experimental results, we propose possible degradation mechanisms related with the role of subgap states. (C) 2011 Elsevier B. V. All rights reserved.