This paper investigates the origin of the bias stability under ambient gas (oxygen, moisture and vacuum) of In-Ga-Zn-O thin film transistors with different annealing temperatures. In Zn-based TFTs, the electrical characteristic of device is a strongly function with the ambient gas, the simultaneous gas ambient and bias stresses are applied on devices annealed in atmosphere ambient to study this issue. The result shows the device which is annealed at temperature up to 330 degrees C has worst reliability. We suppose that the sensitivity of gas ambient depend the defect state, which is associated to the annealing temperature, of surface in a-IGZO. (C) 2011 Elsevier B.V. All rights reserved.