We have studied processing and characteristics of flexible Aluminum-doped Zinc Oxide thin-film transistors (AZO TFTs) fabricated on plastic substrates using radio frequency (rf) magnetron sputtering. To improve the performance of flexible AZO TFT, we studied effects of device structures on characteristics of the aluminum-doped zinc oxide thin film transistors. The electrical properties of top-gate type and bottom-gate type AZO TFTs were investigated, respectively. The top-gate type AZO TFTs shows a threshold voltage of 1.4 V, a Ion/Ioff current ratio of 1.0×107, a field effect mobility of 28.2 cm2/ V•s, a subthreshold swing of 0.19 V/decade. And the bottom-gate type AZO TFTs shows a threshold voltage of 1.7 V, a Ion/Ioff ratio of 1.0×107, a field effect mobility of 209 cm2/ V•s, a subthreshold swing of 0.16 V/decade, and the off current of less than 10-11A at room temperature. Both TFTs show low threshold voltage, high Ion/Ioff ratio and high field effect mobility. By comparison, the bottom-gate type AZO TFTs shows better characteristics. The flexible AZO-TFT is a very promising low-cost optoelectronic device for the next generation of invisible and flexible electronics due to flexible, transparency, high mobility, and low-temperature processing.