Thin film transistors with ZnO as the channel layer and Ta2O5 as the gate dielectric were fabricated by radio frequency (RF) magnetron sputtering. The electrical properties related to the thickness of ZnO active layers, Ta2O5 gate dielectric layer and different annealing times were investigated and the optimal condition was obtained. It reveals that the crystalline quality of ZnO layer is improved by rapid thermal annealing. The optimum device yields an on/off current ratio of 2.4 x 10(4), a threshold voltage of 8.8 V, and a sub-threshold swing of 4.38 V/decade. We contribute the high performance to the proper dielectric thickness, active layer thickness, and proper annealing time. (C) 2013 Elsevier B. V. All rights reserved.