To improve the bias stress stability of ZnO thin film transistors (TFTs), a double-layered Al2O3 gate insulator (GI) prepared by atomic layer deposition was proposed for forming high quality interface and suppressing interdiffusion. The protection layer deposited using water vapor, which also acts as a first GI, plays an important role in improving the bias stability. We obtained excellent characteristics of the ZnO TFT, in which the threshold voltage of 0.65 V and the mobility of 5.87 cm(2) V-1 s(-1) did not show any changes at the stress field of 1.7 MV/cm even after 51,000 s. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3428745] All rights reserved.