We report flexible ZnO thin film transistors (TFTs) fabricated on 5 µm thick solution-cast polyimide substrate. Plasma enhanced atomic layer deposition (PEALD) was used to deposit Al2O3 dielectric and ZnO active layers respectively. The highest processing temperature was 200 °C. The flexible ZnO TFTs we fabricated have very similar characteristics to devices fabricated on glass substrates. Typical TFT mobility was greater than 12 cm2/V·s for a gate electric field of 2 MV/cm. Simple mechanical releasing can be used to detach the polyimide substrate from the rigid carrier. Device performance showed almost no degradation after releasing. Repeated bending test using homemade roller-flex testing apparatus was conducted. The flexible ZnO TFTs were repeatedly bent and flattened to a minimum radius of 1.6 mm for more than 10,000 cycles with little change in device characteristics. These results demonstrated solution-cast thin polymeric film as a simple but profound path to oxide semiconductor flexible electronics.